WASHINGTON, June 9 (Reuters) - U.S. President Barack Obama formally endorsed Hillary Clinton's White House bid on Thursday and called for Democrats to unite behind her after a protracted battle with Bernie Sanders for the party nomination.
U.S. Senator Elizabeth Warren of Massachusetts also backed Clinton on Thursday, telling MSNBC's "The Rachel Maddow Show" that presumptive Republican presidential nominee Donald Trump was "a genuine threat to the country."
Clinton, the presumptive Democratic nominee, said it "means the world" to her that Obama had her back in a bruising campaign for the Nov. 8 election.
See Hillary Clinton through the years:
Hillary Clinton through the years
Obama is 'fired up' for Clinton as Democrats seek to unify party
A 1992 photo shows then Governor of Arkansas Bill Clinton (L) and his wife Hillary (R) embracing. (Photo credit AFP/Getty Images)
Democratic presidential candidate Bill Clinton (L) waves to supporters as he holds the hand of his wife Hillary, 22 July, 1992 after speaking at a rally. St. Louis was the last stop on the Clinton-Gore campaign's bus tour. The crowd was estimated at 40,000. (Photo credit Tim Clary, AFP/Getty Images)
US President Bill Clinton's wife Hillary Rodham Clinton (l) in a picture taken 16 January 1993 in Little Rock, hugs her daughter Chelsea during a farewell address to the people of Arkansas at an airport rally. The Clinton family then left for Charlottesville, Virginia for the start of the planned bus trip to Washington, DC. (Photo credit J. David Ake, AFP/Getty Images)
First Lady Hillary Clinton (R) watches over as 12-year-old Chevon Perry (L) works on a lesson at P.S. 115 Elementary School 26 January 1993. Clinton made her first trip as first lady to New York to receive an award for her service to children. (Photo credit Tim Clary, AFP/Getty Images)
First Lady Hillary Clinton responds to applause, 12 February 1993, in Arlington, VA, as U.S. President Bill Clinton stands behind her. (Photo credit Robert Giroux, AFP/Getty Images)
U.S. First Lady Hillary Rodham Clinton laughs as she is presented a bouquet of flowers by Japanee children 08 July 1993 at the Meguro waste incineration plant in Tokyo, Japan. Mrs. Clinton, maintaining a high profile during the G7 summit, is popular in Japan. (Photo credit David Nelson, AFP/Getty Images)
U.S. President Bill Clinton (R) and First Lady Hillary Rodham Clinton (L) listen to Karl Kregor explain, 16 September 1993 in Washington, D.C. why he is afraid of losng health isurance for his family.The Clintons met at the White House with citizens who shared problems they have had with the present health care system. Clinton is scheduled to present his health care plan to a joint session of congress 22 September. (Photo credit Paul Richards, AFP/Getty Images)
U.S. First Lady Hillary Clinton responds 22 September 1993 to applause from a joint session of the U.S. Congress in Washington, D.C. after President Bill Clinton saluted her as the 'talented navigator' for the national health plan he is proposing. Standing alongside the first lady is Dr. T. Berry Brazelton (L), a noted pediatrician and author. (Photo credit J. David Ake, AFP/Getty Images)
First Lady Hillary Clinton attends the Winter Olympics in Lillehammer, Norway. (Photo by Clive Brunskill, Getty Images)
US First Lady Hillary Clinton (L) and Wife of French Prime Minister Edouard Balladur, Marie-Josephe Balladur pose at the Hotel Matignon on June 7, 1994. (Photo credit Pascal Pavani, AFP/Getty Images)
First Lady Hillary Rodham Clinton (C) smiles at a friend in the crowd 28 September 1994 as she stands next to Russian First Lady Naina Yeltsin (L) during a ceremony at the Library of Congress. Mrs. Yeltsin was accompanying her husband on his visit to Washington for a summit meeting with US President Bill Clinton. AFP YEARENDER (Photo credit Joshua Roberts, AFP/Getty Images)
US President Bill Clinton (R) and First Lady Hillary Clinton leave the White House for the presidential retreat at Camp David, Maryland, 13 Janaury. President Clinton is waving a copy of the morning pool report on his activities, which was written as a poem. The poem begins ' The president jogged at Fort McNair....of him we saw not hide nor hair,' and ends with the slogan 'Burma Shave.' (Photo credit Paul Richards, AFP/Getty Images)
US First Lady Hillary Clinton laughs as she listens to political humorist Bill Maher address the annual dinner of the Radio and Television Correspondents Association in Washington on March 14, 1995. (Photo credit Paul J. Richards, AFP/Getty Images)
US First Lady Hillary Clinton (L) poses with 'Robo Crook'(R) a character from the television program Sesame Street at the White House in Washington, DC 26 June. Mrs. Clinton moderated a panel discussion on the role of Public Television in educating children. (Photo credit Jamal Wilson, AFP/Getty Images)
Hillary Clinton, wife of US President Bill Clinton, talks with Britain's Queen Elizabeth 30 November in the Grand Entrance Hall of Buckingham Palace in London. President Clinton and Hillary Clinton arrived 29 November morning for a three-day visit which will be dominated by the Northern Ireland peace process. (Photo credit John Stillwell, AFP/Getty Images)
US First Lady Hillary Clinton greets guests after speaking at the Women's Leadership Forum at the World Trade Center 20 June in Boston, Massachusetts.The First Lady will be in Washington DC later 20 June for the arrival of the Olympic Torch at the White House. (Photo credit John Mottern, AFP/Getty Images)
US First Lady Hillary Rodham Clinton waves from the bullet-proof presidential limousine as she and US President Bill Clinton leave Sydney International Airport on November 19, 1996. The Clintons arrived in Australia from Hawaii at the start of a five-day working and holiday visit. A(Photo credit should read Torsten Blackwood, AFP/Getty Images)
US President Bill Clinton (L)and his wife Hillary listen to speakers at a coalition for America's Children event at the White House in Washington, on March 3, 1997. The Clintons will begin a series of television, radio and newspaper public service annoucements, urging Americans to help improve the lives of children. (Photo credit Joyce Naltchayan, AFP/Getty Images)
Hillary Clinton proudly shows the tea-pot which had been a gift from community worker Joyce McCartan on the Clintons' previous visit to Belfast, during her speech at the University of Ulster, in Belfast 31 October. The memorial speech, named after Joyce McCartan, who died last year, called on parties from both sides to make compromises for peace in the country. The American First Lady is on a whistle-stop tour visiting Dublin, Belfast and London in three days. (Photo credit Alan Lewis, AFP/Getty Images)
US First Lady Hillary Clinton talks with NBC TV anchorwoman Katie Couric (L)10 September during an event in the East Room of the White House promoting colon cancer awareness and prevention. Hillary Clinton announced a new research grant to fight colon cancer and unveiled a new public service announcement to promote prevention. (Photo credit Tim Sloan, AFP/Getty Images)
US President Bill Clinton, First Lady Hillary Clinton, and their daughter Chelsea tour the Forbidden City in Beijing, as a Chinese security agent (rear) looks on 28 June during their 9 day official trip to China. The three later toured the Great Wall. (Photo credit Paul J. Richards, AFP/Getty Images)
US First Lady Hillary Rodham Clinton listens to speakers during ceremonies 11 January to unveil the new Dolley Madison commemorative silver dollar coin at the White House in Washington, DC. The coin, designed by Tiffany and Company is available at the US Mint and celebrates the life, achievements and 150th anniversary of Dolley Madison's death. (Photo credit Tim Sloan, AFP/Getty Images)
First Lady Hillary Clinton works the crowd as she arrives at Prior Aviation Services in Buffalo, New York, 07 February, 2000. Mrs. Clinton, who officially announced her run for US Senate 06 February, 2000, made Buffalo her fisrt campaign stop. ((Photo credit Don Emmert, AFP/Getty Images)
US First Lady Hillary Rodham Clinton (R) announces the formation of the Millenium Trails Celebration Committee 19 April, 1999 at the Central Park Boathouse in New York, New York. Millenium Trails is a national initiative to create and enhance hiking, cultural and other trails across the country. It partners the White House Millenium Council, the Department of Transportation and the Rails-to-Trails Conservancy. (Photo credit Stan Honda, AFP/Getty Images)
US President Bill Clinton (L) and First Lady Hillary Rodham Clinton (R) wait on the South Portico for the arrival of King Juan Carlos I and Queen Sofia 23 February 2000 at the White House in Washington, DC. The Clintons hosted an arrival ceremony for their visit. (Photo credit Stephen Jaffe, AFP/Getty Images)
US First Lady and New York US Senate candidate Hillary Clinton waves to the crowd as she arrives on the stage at the Democratic National Convention 14 August 2000 at the Staples Center in Los Angeles, California. (Photo credit Paul J. Richards, AFP/Getty Images)
U.S. Senate candidate and First Lady Hillary Rodham Clinton gives a thumbs-up sign to supporters at the Hispanic Day Parade October 8, 2000 in New York City. (Photo by Chris Hondros/Newsmakers via Getty)
US First Lady Hillary Clinton smiles during a press conference in New York 08 November, 2000. Clinton defeated Congressman Rick Lazio to win the US Senate seat for New York being vacated by Patrick Moynihan. AFP PHOTO Doug KANTER (Photo credit Doug Kanter, AFP/Getty Images)
U.S. Senator-elect, First Lady Hillary Clinton walks through the U.S. Capitol during an orientation day for new senators December 5, 2000 in Washington, DC. (Photo by Mark Wilson/Newsmakers via Getty)
Senator Hillary Rodham Clinton listens during Labor Secretary-designate Elaine Chao's confirmation hearing January 24, 2001 on Capitol Hill in Washington, DC. (Photo by Alex Wong/Newsmakers via Getty)
Sen. Hillary Rodham Clinton (D-NY) attends a press conference to address the equal pay act June 12, 2001 Capitol Hill in Washington, DC. The AFL-CIO''s Working Women Working Together held a news conference to introduce a campaign for legislation to step up enforcement of the 1963 Equal Pay Act. (Photo by Alex Wong/Getty Images)
New York Senator and former First Lady, Hillary Rodham Clinton, signs copies of her autobiography, 'Living History' at Waterstone?s bookstore on July 3, 2003 in London, England. In her autobiography, Clinton reveals her presidential ambitions and her thoughts on Monica Lewinsky. (Photo by Scott Barbour/Getty Images)
Senator Hillary Rodham Clinton appears with Jon Stewart during 'The Daily Show With Jon Stewart' at the Daily Show Studios October 8, 2003 in New York City. (Photo by Scott Gries/Getty Images for The Daily Show)
Sen. Hillary Rodham Clinton (D-NY) attends a news conference in front of the United Nations to denounce the International Court of Justice's recent decision on Israel's security fence July 9, 2004 in New York City. The International Court in The Hague has ruled that the barrier Israel has nearly completed in the West Bank violates international law, and the court ruled that the United Nations should take action to stop its construction. (Photo by Spencer Platt/Getty Images)
U.S. Senator Hillary Rodham Clinton (D-NY) speaks while former U.S. President Bill Clinton listens during a visit to the New York Buddhist Vihara December 31, 2004 in the Queens borough of New York City. The Clintons toured the temple where volunteers have been collecting donations for victims of the tsunami in Sri Lanka. (Photo by Stephen Chernin/Getty Images)
Sen. Hillary Rodham Clinton (D-NY) speaks at a Democratic forum on Social Security reform at Pace University March 4, 2005 in New York City. A group of Democratic Senators spoke at the forum to protest U.S. President George W. Bush's plan to privatize Social Security. (Photo by Mario Tama/Getty Images)
U.S. Senator Hillary Clinton (D-NY) stands near her portrait during an unveiling ceremony at the Smithsonian April 24, 2006 in Washington DC. President Clinton's portrait will hang in the National Portrait Gallery's 'America's President's' exhibit. (Photo by Mark Wilson/Getty Images)
Hillary Rodham Clinton watches a speaker on the final day of the Clinton Global Initiative annual meeting September 22, 2006 in New York City. A large array of notables in the worlds of politics, human rights, and philanthropy have gathered in the New York for three days of seminars and pledges on global issues. (Photo by Chris Hondros/Getty Images)
Democratic presidential hopeful Sen. Hillary Clinton (D-NY) speaks during a post primary rally at Southern New Hampshire University January 8, 2007 in Manchester, New Hampshire. After losing to Sen. Barack Obama (D-IL) and Edwards in Iowa, Clinton (D-NY) won New Hampshire, the first of the nation's presidential primaries. (Photo by Chris Hondros/Getty Images)
US Senator Hillary Clinton and daughter Chelsea are seen at the lectern during a soundcheck at the Democratic National Convention 2008 at the Pepsi Center in Denver, Colorado, on August 26, 2008. Clinton takes the stage tonight vowing to unite Democrats after her primary battle with Barack Obama, on the second day of the convention that will crown him as White House nominee. The DNC is held 25-28 August. (Photo credit Stan Honda, AFP/Getty Images)
Democratic presidential nominee U.S. Sen. Barack Obama (D-IL) and Sen. Hillary Clinton (D-NY) attend a campaign rally together at Amway Arena October 20, 2008 in Orlando, Florida. Obama continues to campaign against his challenger, Republican presidential nominee Sen. John McCain (R-AZ) while Election Day begins to draw near. (Photo by Joe Raedle/Getty Images)
Former US President Bill Clinton (L) and US Secretary of State-designate Hillary Clinton (R) attend the inauguration of US President Barack Obama at the US Capitol in Washington, DC, on January 20, 2009. (Photo credit Robyn Beck, AFP/Getty Images)
U.S. Secretary of State Hillary Clinton (L) is sworn in as her husband former President Bill Clinton (2nd L), and her daughter Chelsea (R) look on during a ceremonial swearing-in at the State Department February 2, 2009 in Washington, DC. Clinton is the 67th Secretary of State of the United States of America. (Photo by Alex Wong/Getty Images)
US Secretary of State Hillary Clinton attends a press conference on February 21, 2009 in Beijing, China. Clinton is on a three day visit to the Chinese capital, as part of her first diplomatic tour to Asia. (Photo by Guang Niu/Pool/Getty Images)
US Secretary of State Hillary Clinton speaks on the US vision for Asia-Pacific multilateral engagement at the East-West Center in Honolulu January 12, 2010. (Photo credit Mandel Ngan, AFP/Getty Images)
US Secretary of State Hillary Clinton, smiles during a press conference after Middle East Quartet talks in Moscow on March 19, 2010. The International Quartet on the Middle East urged Israel to freeze all settlement activity and expressed deep concern about the situation in Gaza, UN Secretary General Ban Ki-moon said. (Photo credit Yuri Kadobnov, AFP/Getty Images)
US Secretary of State Hillary Clinton speaks before signing the US-China Consultation on People-to-People Exchange agreement at the National Center for the Performing Arts in Beijing on May 25, 2010. The United States and China were wrapping up strategic talks aimed at smoothing out differences on currency and trade issues, as Washington presses Beijing to get tough on North Korea. (Photo credit Saul Loeb, AFP/Getty Images)
U.S. Secretary of State Hillary Clinton listens to Ambassador-Designate to Russia Michael McFaul during his swearing-in ceremony at the State Department January 10, 2011 in Washington, DC. McFaul is President Barack Obama's top adviser on Russia and has been involved in the reset of relations between the two countries and the signing of the New START treaty. (Photo by Astrid Riecken/Getty Images)
U.S. Secretary of State Hillary Clinton speaks during the South Sudan International Engagement Conference December 14, 2011 at the Marriott Wardman Park Hotel in Washington, DC. The two-day conference was to highlight the national development vision of South Sudan and the opportunities for investment in the country. (Photo by Alex Wong/Getty Images)
US Secretary Hillary Clinton speaks to reporters during a joint press conference with Qatar's Prime Minister and Foreign Minister Hamad bin Jassim bin Jabr Al-Thani at the Department of State January 11, 2012 in Washington, DC. (Photo credit Karen Bleier, AFP/Getty Images)
US Secretary of State Hillary Clinton (R) shakes hands with Moroccan Foreign Minister Saad-Eddine el-Othmani prior to meetings at the State Department in Washington, DC, on March 15, 2012. (Photo credit Saul Loeb, AFP/Getty Images)
US Secretary of State Hillary Clinton smiles during a joint press conference with Australian Foreign Minister arr following their meeting at the State Department in Washington, DC, on April 24, 2012. (Photo credit Jewel Samad, AFP/Getty Images)
U.S. Secretary of State Hillary Clinton testifies before the House Foreign Affairs Committee on Capitol Hill January 23, 2013 in Washington, DC. Lawmakers questioned Clinton about the security failures during the September 11 attacks against the U.S. mission in Benghazi, Libya, that led to the death of four Americans, including U.S. Ambassador Christopher Stevens. (Photo by Chip Somodevilla/Getty Images)
Former US Secretary of State Hillary Clinton addresses the Vital Voices Global Awards ceremony at the Kennedy Center in Washington on April 2, 2013. The event honors 'women leaders from around the world who are the unsung heroines to strengthen democracy, increase economic opportunity, and protect human rights,' according to the group's website. (Photo credit Nicholas Kamm, AFP/Getty Images)
Former US Secretary of State Hillary Clinton speaks about the situation in Syria after meeting with US President Barack Obama, prior to remarks about US efforts to combat wildlife trafficking at a White House forum at the Eisenhower Executive Office Building in Washington, DC on September 9, 2013. (Photo credit Saul Loeb, AFP/Getty Images)
Former Secretary of State Hillary Rodham Clinton speaks on stage at the Pennsylvania Conference For Women 2013 at Philadelphia Convention Center on November 1, 2013 in Philadelphia, Pennsylvania. (Photo by Lisa Lake/Getty Images for Pennsylvania Conference for Women)
Hillary Clinton attends the New-York Historical Society 2014 History Makers Gala at Mandarin Oriental Hotel on November 21, 2014 in New York City. (Photo by Brad Barket/Getty Images)
Hillary Rodham Clinton Book Signing For 'Hard Choices' at Barnes & Noble bookstore at The Grove on June 19, 2014 in Los Angeles, California. (Photo by Frazer Harrison/Getty Images)
Former U.S. Secretary of State Hillary Clinton delivers a keynote address during the Watermark Silicon Valley Conference for Women on February 24, 2015 in Santa Clara, California. Former Secretary of State Hillary Clinton delivered a keynote address to thousands of women in attendance for the Watermark Silicon Valley Conference for Women. (Photo by Justin Sullivan/Getty Images)
Former Secretary of State Hillary Clinton speaks on stage during a ceremony to induct her into the Irish America Hall of Fame on March 16, 2015 in New York City. The Irish America Hall of Fame was founded in 2010 and recognizes exceptional figures in the Irish American community. (Photo by Yana Paskova/Getty Images)
Discover More Like This
BACK TO SLIDE
Clinton also said she had the "highest regard" for Warren, a fiery critic of Wall Street, and was "really pleased to have her good ideas and support."
Vice President Joe Biden also waded into the campaign on Thursday. "Whoever the next president is, and God willing in my view it will be Secretary Clinton," Biden said in a speech at the American Constitution Society in Washington.
The Obama endorsement increases pressure on Sanders, a U.S. senator from Vermont, to bow out of the race and lend his support to Clinton so that the party can focus on defeating Trump.
"It is absolutely a joy and an honor that President Obama and I over the years have gone from fierce competitors to true friends," Clinton told Reuters in an interview.
After an unexpectedly tough battle against Sanders' challenge from the left, former first lady Clinton made history when she reached the number of delegates needed to win the party nomination this week. That made her the first woman to lead a major U.S. party as its White House candidate.
Obama, who enjoys rising approval ratings as he nears the end of eight years in office, will appear with Clinton on the campaign trail next week in Wisconsin.
The two were opponents in the 2008 Democratic presidential primary race, which Obama won, but they buried their rivalry and she served as his secretary of state for four years. Clinton is the 2016 candidate who the White House believes will best safeguard Obama's legacy.
"I don't think there's ever been someone so qualified to hold this office," Obama said of Clinton in a video. "I'm with her. I am fired up, and I cannot wait to get out there and campaign for Hillary."
Trump assailed the endorsement on Twitter: "He wants four more years of Obama-but nobody else does!"
Clinton's campaign tweeted a brash response: "Delete your account."
Sanders, who galvanized young voters with his calls for more social equality and measures to rein in Wall Street, has been reluctant to concede the race, despite concerns among leading Democrats that continuing party divisions could hamper Clinton's efforts to beat Trump.
TOWARD THE EXIT
Obama and other senior Democrats are seeking a delicate balance of rallying the party behind Clinton, while not alienating Sanders and his supporters.
In what appeared to be an attempt to gently ease Sanders toward giving up his campaign, Obama met the democratic socialist for about an hour in the White House, laughing warmly as they walked into the Oval Office.
Although Sanders told reporters afterward that he still planned to compete in the final nominating contest in Washington, D.C., next Tuesday, he said he would work with Clinton to defeat Trump.
Sanders was then welcomed on Capitol Hill by Senator Harry Reid, the top Democrat in the Senate. Reid said the lawmaker from Vermont was in a "good place" with his Democratic colleagues. He suggested that Sanders was close to acknowledging defeat by Clinton.
"I didn't hear a single word about him trying to change the fact that she is the nominee, I think he's accepted that," Reid told reporters.
In the endorsement video, Obama recalled the party unity that followed his prolonged primary battle against Clinton in 2008.
"Secretary Clinton and Senator Sanders may have been rivals during this primary, but they're both patriots who love this country and they share a vision for an America that we all believe in," Obama said.
Warren told MSNBC she was endorsing Clinton because "a female fighter in the lead is exactly what this country needs."
Warren's populist credentials will boost Clinton's ability to court Sanders voters as she prepares to battle Trump. Warren was the only female Democratic U.S. senator who did not endorse Clinton during the primary race.
Clinton told Reuters she and Warren had similar views about issues such as economic policy and protecting the 2010 Dodd-Frank financial reform law and the Consumer Financial Protection Bureau, which Warren pushed to start.
Trump said in an interview with Reuters last month that he would try to dismantle the Dodd-Frank law.
In the interview with Reuters, Clinton said her overall economic package, including plans to rein in Wall Street and cut taxes for the middle class, would come during the first 100 days of her presidency if she defeats Trump.
Clinton previously said a plan to generate jobs by investing in transportation and other infrastructure spending and immigration reform would be among other early priorities.
"One of the things that President Obama said yesterday is he thought his job was to remind the American people what a really serious job this is, the tough choices, the hard decisions, the high stakes in choosing a president and commander in chief," Clinton said.
"And I know how important it is to get off to a really good start in the White House," she said.
Trump, a wealthy real estate developer who became the party's presumptive nominee last month after seeing off a large group of rivals, is well behind Clinton's campaign in terms of fundraising and policy infrastructure.
On Thursday, his top donors were holding their first official meeting in New York. Trump also met with industry leaders in New York at an event organized by oil billionaire Harold Hamm.