Joe Biden has tested positive for Covid-19 and is suffering mild symptoms, the White House says.
Karine Jean-Pierre, his press secretary, said the US president was vaccinated and boosted. He has tested positive for Covid twice before.
Mr Biden, 81, was seen earlier on Wednesday visiting supporters in Las Vegas and speaking at an event. He cancelled a campaign speech later in the night.
The illness comes as he faces increasing pressure to step aside because of his age.
US media report that both Senate Majority Leader Chuck Schumer and House Majority Leader Hakeem Jefferies – the top two Democrats in the US Congress – separately met Mr Biden privately and expressed deep concern that his candidacy could negatively impact other House and Senate races.
Ms Jean-Pierre said the president planned to isolate at his home in Delaware while carrying out “all of his duties fully”.
The president’s doctor, Kevin O’Connor, said Mr Biden had presented with upper respiratory symptoms, including a runny nose and a cough, and had been given his first dose of Paxlovid.
He felt fine during his first event of the day but later tested positive, Dr O’Connor said.
Mr Biden later used X/Twitter to thank everyone for “the well wishes” and said he would “work to get the job done for the American people” while in recovery.
In another tweet his account stated “I’m sick” before replying back “… of Elon Musk and his rich buddies trying to buy this election. And if you agree, pitch in here”.
The tweet pointed to a donations portal.
Earlier on Wednesday, reporters on the Las Vegas trip said they had been rushed off the site of a campaign stop at a Mexican restaurant to the city’s airport following the announcement.
The restaurant was ablaze with colour, with Mexican art and guitars hanging from the walls and banners draped from the ceiling. One wall was adorned with “Biden-Harris” posters.
As ceiling fans swirled on low speed and Latino pop played quietly from the speakers, Mr Biden walked in via the kitchen door, which was flanked by a Secret Service detail, and entered a main dining area.
He shook hands with diners (who had clearly been prepared some time in advance for his arrival), kissed one and had selfies taken with others.
The president seemed somewhat stiffer and appeared slower than the day before, when he had given an energised performance in a speech to a national civil rights group.
Mr Biden was forced to cancel a speech at UnidosUS, a Latino civil rights organisation.
The president has been trying to revive his appeal among Hispanic voters, with whom his poll lead has slipped since 2020.
Later on Wednesday, he was seen moving slowly and cautiously up the steps to the plane. He was not wearing a mask.
As he boarded Air Force One he was heard to say: “Good, I feel good.”
It is a sudden and debilitating end to a trip that was supposed to see Mr Biden hitting the campaign trail again with full force – after the pause sparked by the attempted assassination of his rival Donald Trump.
Mr Biden’s re-election attempt was already being buffeted by growing dissent among Democrats in Congress after his disastrous debate performance against Donald Trump last month.
And now it is hitting even more turbulence as Air Force One bounces its way through the hot air above Nevada and the president up front is struck by another unexpected blow.
This election campaign feels like it is moving at a chaotic, unpredictable pace.
Mr Biden’s illness comes as he faces growing calls to withdraw from the election race.
Nearly two dozen Democratic politicians have called for him to step aside in recent weeks, including Adam Schiff, a congressman from California, who said he had serious doubts about whether the president could beat former Mr Trump.
He called on Mr Biden to “pass the torch”.
Mr Schiff said Mr Biden “has been one of the most consequential presidents in our nation’s history”, and he could “secure his legacy of leadership” by allowing another Democrat to step forward.
Mr Schumer and Mr Jeffries – Congress’ top two Democrats – met Mr Biden privately in recent days and expressed concerns among fellow lawmakers that his presence at the top of the November election ticket could hurt their chances for controlling either chamber in Congress, according to reports from ABC News, the Washington Post and Politico.
“The president told both leaders he is the nominee of the party, he plans to win, and looks forward to working with both of them to pass his 100 days agenda to help working families,” White House spokesman Andrew Bates said after the reports.
A spokesman for Mr Jeffries said, “it was a private conversation that will remain private”. Mr Schumer’s office called the reporting “idle speculation” but added the Democratic leader “conveyed the views of his caucus directly to President Biden”.
In an interview with BET, Mr Biden said he did not feel he could pass the mantle with the country so “divided”.
The president also said, for the first time, that he would consider dropping out of the race if any of his doctors said he had a “medical condition”.