Joe Biden has tested positive for Covid-19 and is suffering mild symptoms, the White House has said.
Karine Jean-Pierre, his press secretary, said the US president is 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 has 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 reports both Senate Majority Leader Chuck Schumer and House Majority Leader Hakeem Jefferies – the top two Democrats in the US Congress – separately met with Mr Biden privately and expressed that there is deep concern 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 he carries out “all of his duties fully during that time”.
The president’s doctor, Kevin O’Connor, said Mr Biden presented with upper respiratory symptoms, including a runny nose and a cough and was 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.
Reporters on the Las Vegas trip said they were rushed to the city’s airport following the announcement.
Mr Biden moved slowly and cautiously up the steps to the plane, video shows. He was not wearing a mask.
As he boarded Air Force One he was heard to say: “Good, I feel good.”
The president was forced to cancel a speech at UnidosUS, a Latino civil rights organisation.
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 today he had serious doubts about whether the president could beat former President Donald Trump.
He called on Mr Biden to “pass the torch”.
Mr Schiff said that 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 with Mr Biden privately in recent days and expressed concerns by fellow lawmakers that him being 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, which was due to be broadcast on Wednesday evening, 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”.