Joe Biden is ill with Covid-19, aides said, as the president insisted that only ill-health would force him out of the White House race against Donald Trump.
The 81-year-old, who faces ever-rising pressure from fellow Democrats to bow out, was said to have tested positive while travelling in Las Vegas on Wednesday and was experiencing "mild symptoms".
Mr Biden flew back to his Delaware home where he will "self-isolate and will continue to carry out all of his duties fully during that time", White House press secretary Karine Jean-Pierre said.
The president was said by White House physician Dr Kevin O'Connor to have upper respiratory symptoms, a runny nose, a cough and “general malaise”.
But gingerly boarding Air Force One, Mr Biden told travelling reporters: "I feel good." He was not wearing a mask.
The Democrat is the oldest person yet to serve as US president. He has been under a particularly glaring spotlight over his physical and mental health for weeks since an embarrassing debate performance against Mr Trump.
Californian Representative Adam Schiff, who is close to former House Speaker Nancy Pelosi, became one of the most high-profile party figures yet to call on the president to drop out of the battle against the Republicans.
Mr Schiff said in a statement: "While the choice to withdraw from the campaign is President Biden's alone, I believe it is time for him to pass the torch. And in doing so, secure his legacy of leadership by allowing us to defeat Donald Trump in the upcoming election.”
Mr Biden appeared to zone out again at times in an interview with BET News journalist Ed Gordon. But he was clear when asked if there were any factors that would make him reconsider running for the November 5 election.
“If I had some medical condition that emerged,” Mr Biden said.
“If doctors came to me and said, ‘you got this problem, that problem’.”But he added: "I think I've demonstrated that I know how to get things done for the country in spite of the fact that we were told we couldn't get it done… But there's more to do, and I'm reluctant to walk away from that."
The president said at a news conference last week that none of his doctors have told him he has any new medical condition, and he told Mr Gordon that his "serious mistake in the whole debate" should not disqualify him from continuing to serve.
"When I originally ran, you may remember Ed, I said I was going to be a transitional candidate, and I thought that I would be able to move from this and pass it on to someone else," he said.
"But I didn't anticipate things getting so, so, so divided. And quite frankly, I think the only thing age brings is a little bit of wisdom."
However, both in public and in private, senior Democrats have been sending signals of concern as the party prepares for its nominating convention in Chicago in four weeks.
The top-ranking Democrats in Congress, Senator Chuck Schumer and Representative Hakeem Jeffries, are both said to have spoken privately to the president to argue that he cannot beat Mr Trump.
Mr Schumer and Mr Jeffries have already intervened to tell Democratic national committee chiefs to delay a much-criticised plan to hold a virtual vote to renominate Mr Biden and Vice President Kamala Harris as early as Sunday, well ahead of the Chicago convention.
Ms Pelosi has also reportedly intervened with Mr Biden privately, and former president Barack Obama is said to be similarly concerned about November.
Critics have highlighted polling that shows Mr Biden trailing Mr Trump in all the major battleground states - even as Democratic candidates for governor and senator are seen beating their Republican rivals in the same states.
Among Democrats nationwide, nearly two-thirds say Mr Biden should step aside and let his party nominate a different candidate, according to a bombshell poll by AP-NORC Center for Public Affairs Research.
