Trump signs pledge to back GOP's 2016 presidential nominee

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Donald Trump Pledges He Won't Run As An Independent

WASHINGTON (AP) -- Presidential candidate Donald Trump ruled out the prospect of a third-party White House bid Thursday and vowed to support the Republican Party's next presidential nominee - whoever it may be.

The billionaire businessman announced his decision in a raucous new conference at Trump Tower, the gold-hued skyscraper in midtown Manhattan where he launched his surging and front-running campaign for president.

"The best way forward ... to win, is if I win the nomination and go direct against whoever (the Democrats) happen to put up. So for that reason, I have signed the pledge," Trump said.

RELATED: GOP asks all candidates to sign binding pledge

"So, I will be totally pledging my allegiance to the Republican Party and the conservative principles for which it stands, and we will go out and we will fight hard and we will win," he said.

See the thousands of people who came out to see Trump in Alabama:

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Trump signs pledge to back GOP's 2016 presidential nominee
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump speaks during a rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump speaks during a rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump speaks during a rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump greets supporters after his rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump greets supporters after his rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump greets supporters after his rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
Republican presidential candidate Donald Trump speaks during a campaign rally in Mobile, Ala., on Friday, Aug. 21, 2015. (AP Photo/Brynn Anderson)
The crowd cheers when Republican presidential candidate businessman Donald Trump speaks during a campaign pep rally, Friday, Aug. 21, 2015, in Mobile, Ala. (AP Photo/Brynn Anderson)
Republican presidential candidate Donald Trump speaks during a campaign pep rally, Friday, Aug. 21, 2015, in Mobile, Ala. (AP Photo/Brynn Anderson)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump speaks during a rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
Buttons supporting Republican presidential candidate businessman Donald Trump are displayed at a campaign pep rally, Friday, Aug. 21, 2015, in Mobile, Ala. (AP Photo/Brynn Anderson)
MOBILE, AL- AUGUST 21: Republican presidential candidate Donald Trump speaks during a rally at Ladd-Peebles Stadium on August 21, 2015 in Mobile, Alabama. The Trump campaign moved tonight's rally to a larger stadium to accommodate demand. (Photo by Mark Wallheiser/Getty Images)
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Trump's decision comes weeks after he roiled the race for the GOP nomination when, in response to the first question at the opening debate of the 2016 campaign, he refused to promise to back the party's nominee if he fell short.

He was intensely lobbied by Republican National Committee leaders, who have struggled to rein in the unpredictable former reality television star. Trump announced his decision shortly after meeting privately with RNC Chairman Reince Priebus.

The decision puts an end - for now - to the nervousness felt inside the GOP about the prospect of Trump holding firm and keeping his options open. At the debate, he said that gave him "a lot of leverage."

The pledge is not legally binding. Trump could always change his mind, particularly if GOP establishment leaders take aggressive steps to thwart his candidacy in the coming months.

"I see no circumstances under which I would tear up that pledge," Trump said Thursday.

If not for Trump, the need for such a loyalty oath probably would not exist. There were no doubts about the intentions of the GOP's other major presidential contenders headed into the debate, and they quickly lined up Thursday to sign.

See how Trump's look has evolved through the years:

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Donald Trump through the years
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Trump signs pledge to back GOP's 2016 presidential nominee
Real estate developer Donald Trump annouces intentions to build a $100 million dollar Regency Hotel. (Photo by John Pedin/NY Daily News via Getty Images)
UNITED STATES - MARCH 02: Donald Trump with Alfred Eisenpreis, New York City Economic Development Administrator. Sketch of new 1,400 room Renovation project of Commodore Hotel. (Photo by NY Daily News Archive via Getty Images)
NEW YORK, NY - 1980: Donald Trump and Ivana Trump attend Roy Cohn's birthday party in February 1980 in New York City. (Photo by Sonia Moskowitz/Getty Images)
UNITED STATES - MAY 26: Donald Trump stands behind architect's model of City Hall Plaza. (Photo by Frank Russo/NY Daily News Archive via Getty Images)
Donna Mills and Donald Trump during 1983 Annual American Image Awards at Sheraton Center in New York City, New York, United States. (Photo by Ron Galella, Ltd./WireImage)
Portrait of real estate mogul Donald John Trump (b.1946), smiling slightly and facing to his right, 1983. New York. (Photo by Bachrach/Getty Images)
New York real estate magnates Steve Ross, right, and Donald Trump, left, announce agreement, Thursday, August 1, 1985 in New York, to merge the Houston Gamblers and the New Jersey Generals United States Football League teams. Ross heads a group of investors that last week agreed to buy the troubled Houston franchise. (AP Photo/Marty Lederhandler)
Real estate magnate Donald Trump poses in front of one of three Sikorsky helicopters at New York Port Authority's West 30 Street Heliport on March 22, 1988. (AP Photo/Wilbur Funches)
Ivana Trump and Donald Trump during Mike Tyson vs Michael Spinks Fight at Trump Plaza - June 27, 1988 at Trump Plaza in Atlantic City, New Jersey, United States. (Photo by Ron Galella/WireImage)
Donald Trump and his wife, Ivana, pose outside the Federal Courthouse after she was sworn in as a United States citizen, May 1988. (AP Photo)
NEW YORK, NY - DECEMBER 4: Billionaire Donald Trump and his wife Ivana arrive 04 December 1989 at a social engagement in New York. (Photo credit should read SWERZEY/AFP/Getty Images)
Shown in photo is Donald Trump, Nov. 20, 1990. (AP Photo/Mark Lennihan)
Billionaire developer Donald Trump, right, waits with his brother Robert for the start of a Casino Control Commission meeting in Atlantic City, N.J., March 29, 1990. Trump was seeking final approval for the Taj Mahal Casino Resort, one of the world's largest casino complexes. (AP Photo)
Developer Donald Trump, center, is flanked by super middleweight champion Thomas Hearns, left, of Detroit, and Michael Olajide of Canada at a news conference in New York Thursday, Feb. 15, 1990. The three announced the super middleweight title bout at Trump Taj Mahal Casino Resort at Atlantic city, N. J on April 28.(AP Photo/Timothy Clary)
Real estate magnate Donald Trump and his girlfriend Marla Maples are seen at the Holyfield-Foreman fight at Trump Plaza in Atlantic City, N.J., April 19, 1991. (AP Photo)
Donald Trump and Daughter Ivanka Trump during Maybelline Presents 1991 Look of the Year at Plaza Hotel in New York City, New York, United States. (Photo by Ron Galella/WireImage)
NEW YORK, NY - APRIL 7: Donald Trump touches 07 April 1993 Marla Maples stomach to confirm published reports that the actress is pregnant with his child. The two arrived for Maples appearance in the Broadway musical 'The Will Rogers Follies'. (Photo credit should read HAI DO/AFP/Getty Images)
NEW YORK, NY - DECEMBER 21: US business tycoon Donald Trump(C) enters the PLaza Hotel in New York past supporters 21 December 1994. Hundreds of supporters showed up at a news conference where Trump denied a New York newspaper report that the Sultan of Brunei had bid 300 million USD to buy the Manhattan hotel. (Photo credit should read DON EMMERT/AFP/Getty Images)
FILE--This is a 1994 file photo of Donald Trump. Trump said Wednesday, Oct. 23, 1996 he has bought the Miss Universe, Miss USA and Miss Teen beauty pageants from ITT. ``It's a done deal,'' Trump said in a telephone interview. ``It's a very, very great entertainment format. It gets very high ratings, it's doing very well and we'll make it even better.'' Trump declined to say how much he paid. Asked if a New York Post source was correct in saying the deal was worth tens of millions of dollars, Trump replied, ``Why not? (AP Photo/Jim Cooper)
FLUSHING MEADOWS, UNITED STATES: Donald Trump and his girlfriend Celina Midelfar watch Conchita Martinez and Amanda Coetzer 07 September at US Open in Flushing Meadows, NY. AFP PHOTO Timothy CLARY (Photo credit should read TIMOTHY CLARY/AFP/Getty Images)
Donald Trump and Christine Whitman during Opening of New Warner Bros. Store in Trump Plaza Casino at Trump Plaza Casino in Atlantic City, New Jersey, United States. (Photo by Ron Galella/WireImage)
In this June 7, 1995 file photograph, Donald Trump is seen above the floor of the New York Stock Exchange after taking his flagship Trump Plaza Casino public in New York City. Trump Entertainment Resorts Inc., based in Atlantic City, New Jersey, filed for Chapter 11 protection on Tuesday, Feb. 17, 2009, in the U.S. Bankruptcy Court in New Jersey. Trump and his daughter Ivanka resigned from the company's board Friday, Feb. 13, 2009, after growing frustrated with bondholders. (AP Photo/Kathy Willens,File)
Celine Dion, husband Rene, Donald Trump & Ivanka Trump (Photo by KMazur/WireImage)
Entrepreneur Donald Trump watches an undercard fight as an unidentified companion whispers into his ear before the start of the Mike Tyson versus Francois Botha bout at the MGM Grand Hotel & Casino in Las Vegas, Nevada, Saturday, Jan. 16, 1999. (AP Photo/Eric Draper)
Developer Donald Trump holds an umbrella as he walks Saturday, Nov. 9, 2002, to the 11th green of the Ocean Trail Golf Club in Rancho Palos Verdes, Calif. Trump plans to turn the beleagured golf club into a world class course. Trump intends to close on the golf club by December and hopes to begin improvements by January. He could reopen the course, 20 miles south of Los Angeles, as early as June. (AP Photo/Damian Dovarganes)
Entrepreneur Donald Trump (L) and Rev. Al Sharpton speak at a ribbon cutting ceremony for Sharpton's National Action Network Convention April 5, 2002 in New York City. The group aims to further the development of civil rights. (Photo by Mario Tama/Getty Images)
Donald Trump and his girlfriend Melania Knauss attend the Marc Bouwer/Peta Fall/Winter 2002 Collection show February 14, 2002 during Mercedes-Benz Fashion Week in New York City. (Photo by George De Sota/Getty Images)
NEW YORK, UNITED STATES - DECEMBER 06: WBC Kampf im Schwergewicht 2003, New York/Madison Square Garden; Vitali KLITSCHKO/UKR - Kirk JOHNSON/CAN; Donald TRUMP als Zuschauer (Photo by Alexander Hassenstein/Bongarts/Getty Images)
FOXBORO, MA - JANUARY 10: Donald Trump stands on the sidelines before the start of the AFC divisional playoffs between the New England Patriots and Tennessee Titans on January 10, 2004 at Gillette Stadium in Foxboro, Massachusetts. Temperatures have reached as low as 7 degrees in the Foxboro area. (Photo by Al Bello/Getty Images)
US tycoon Donald Trump arrives to speak at the annual Conservative Political Action Conference (CPAC) at National Harbor, Maryland, outside Washington, on February 27, 2015. AFP PHOTO/NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
NASHVILLE, TN - APRIL 10: Donald Trump speaks during the NRA-ILA Leadership Forum at the 2015 NRA Annual Meeting & Exhibits on April 10, 2015 in Nashville, Tennessee. The annual NRA meeting and exhibit runs through Sunday. (Photo by Justin Sullivan/Getty Images)
WASHINGTON, DC - MARCH 24: Real estate mogul and billionaire Donald Trump attends Golf legend Jack Nicklaus' Congressional Gold Medal ceremony in the U.S. Capitol Rotunda March 24, 2015 in Washington, DC. Trump announed on March 18 that he has launched a presidential exploratory committee. (Photo by Chip Somodevilla/Getty Images)
DES MOINES, IA - MAY 16: Businessman Donald Trump speaks to guests gathered for the Republican Party of Iowa's Lincoln Dinner at the Iowa Events Center on May 16, 2015 in Des Moines, Iowa. The event sponsored by the Republican Party of Iowa gave several Republican presidential hopefuls an opportunity to strengthen their support among Iowa Republicans ahead of the 2016 Iowa caucus. (Photo by Scott Olson/Getty Images)
Developer Donald Trump displays a copy of his net worth during his announcement that he will seek the Republican nomination for president, Tuesday, June 16, 2015, in the lobby of Trump Tower in New York. (AP Photo/Richard Drew)
Republican presidential hopeful Donald Trump gives a thumbs up before boarding his campaign plane to depart from Laredo, Texas, Thursday, July 23, 2015. (AP Photo/LM Otero)
AYR, SCOTLAND - JULY 30: Republican Presidential Candidate Donald Trump visits his Scottish golf course Turnberry with his children Ivanka Trump and Eric Trump on July 30, 2015 in Ayr, Scotland. Donald Trump answered questions from the media at a press conference. (Photo by Jeff J Mitchell/Getty Images)
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"The RNC clearly felt it had to box Trump into a decision," said Doug Watts, a spokesman for fellow candidate and retired surgeon Ben Carson. "We just sort of shrugged our shoulders, and that's the end of that."

A third-party bid by Trump could harm the GOP's efforts to take back the White House after eight years of Democratic President Barack Obama. He leads the Republican field in what are still very early polls.

"Ross Perot ended up giving us the first Clinton, and I think if Trump does this, he could give us another Clinton," Kentucky Sen. Rand Paul said on "CBS This Morning," referring to Democratic front-runner Hillary Rodham Clinton and her husband, former President Bill Clinton.

RNC officials have been working with Trump's campaign to avoid such a scenario. In recent days, Trump hinted the lobbying was beginning to work. "I think a lot of people are going to be very happy," he said Saturday in Tennessee.

The RNC's pledge asks candidates to promise to "endorse the 2016 Republican presidential nominee regardless of who it is." Further, it asks them to pledge "that I will not seek to run as an independent or write-in candidate, nor will I seek or accept the nomination for president of any other party."

"It is, more than anything, your word," former technology executive Carly Fiorina said Thursday on CNN's "New Day." `'And I would presume that somebody running for president would like to signal to the American people, and most especially right now to Republican primary voters, that their word can be trusted."

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