US 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.
US 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.
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 US 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 US 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.
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.