In an interview with Reuters Clinton said, "It is absolutely a joy and an honor that President Obama and I, over the years, have gone from fierce competitors to true friends."
Obama beat out Clinton during the 2008 Democratic primary. Clinton went on to serve as his secretary of state during his first term in office.
Who will Clinton's running mate be? See a list of potential options below:
Hillary Clinton potential running mates, VPs
President Obama endorses Hillary Clinton for president
The junior Democratic Senator from the swing state of Virginia could be a strategic selection for Hillary. Kaine also served as the governor of Virginia from 2006- 2010.
(Photo by Samuel Corum/Anadolu Agency/Getty Images)
The current U.S. Senator from Massachusetts is popular among progressive Democrats, and some even tried to draft her to run for president herself in 2016.
(Photo by Andrew Harrer/Bloomberg via Getty Images)
Insiders believe that the senior U.S. Senator from Ohio could help Clinton increase her popularity with working-class voters, a group she has yet to win in a big way so far in primary contests.
(Photo By Bill Clark/CQ Roll Call)
The U.S. Senator from New Jersey is both youthful and charismatic and would add racial diversity to a Clinton ticket.
(Photo by KK Ottesen for The Washington Post via Getty Images)
The current U.S. Secretary of Labor is considered a sleeper pick by many Democrats because he is not well known outside of D.C., but some believe his strength and popularity among union workers and other progressive groups could be an asset to Clinton's ticket.
(Photo By Tom Williams/CQ Roll Call)
The Independent from Vermont has become Hillary Clinton's primary rival for the Democratic nomination, garnering a surprising amount of support. Bringing Sanders onto the ticket could help to unite both sets of supporters who have been split in Democratic primaries.
(Photo by Ralph Freso/Getty Images)
A former 2016 rival of Hillary Clinton, and former Maryland governor, Martin O’Malley could help bring some executive experience, along with a slight youthful boost to the ticket.
(Photo by Joe Raedle/Getty Images)
The Secretary of Agriculture since 2009, Tom Vilsack also served as the governor of Iowa from 1999 to 2007. Vilsack could bring some governing experience along with swing state influence.
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Evan Bayh could bring a more right leaning brand of politics to the ticket. Bayh previously served as the junior U.S. Senator from Indiana from 1999 to 2011, and also as the 46th Governor of Indiana from 1989 to 1997.
While the likelihood of him agreeing to take on the veep job again might be low, Biden's popularity among Democrats would likely boost Clinton's chances.
(Photo credit MANDEL NGAN/AFP/Getty Images)
Hillary's husband is technically allowed to serve in the job, and some legal experts even think he'd be able to take office if necessary. Unfortunately for the diehard Clinton supporters, a Clinton-Clinton ticket will probably be a dream that never comes true.
(Photo By Bill Clark/CQ Roll Call)
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White House spokesman Josh Earnest said Thursday in his daily press briefing Clinton's rival, Sen. Bernie Sanders, would not be taken off guard by the endorsement.
"The president has had the opportunity to speak to Senator Sanders now three times in the last week, and as a result of those conversations, I think it's fair to say that Senator Sanders was not at all surprised by today's announcement," Earnest said.
Obama met with Sanders at the White House Thursday morning to discuss the Vermont senator's future plans. Clinton declared victory over Sanders on Tuesday after securing a majority of pledged delegates. Sanders vowed to work with his Democratic rival to defeat Republican Donald Trump.
While the move puts increasing pressure on Sanders to concede the nomination so the party can unify behind Clinton, Earnest said Sanders had "earned the right to make his own decision" on when to end his campaign. Sanders said earlier Thursday he would stay in the race to compete in the final Democratic primary vote in Washington, D.C., on June 14.
Senate Democratic leader Harry Reid said Thursday that Sanders had accepted that Clinton is the Democratic Party's nominee, but "needs a little time" to decide his next step.