While rival Donald Trump prepared for his star turn at the Republican National Convention, the 81-year-old US president found himself in both personal and political isolation.
The top Democrats in Congress, Senate Majority Leader Chuck Schumer and House Minority Leader Hakeem Jeffries, both reportedly met with Biden in recent days to warn that his candidacy threatens his party's prospects in November's election.
Influential former House speaker Nancy Pelosi added to his woes by privately telling Biden he cannot win and could harm Democrats' chances of recapturing the lower chamber, CNN reported.
Several party figures were meanwhile quoted anonymously by the Axios news outlet as saying that they believed the pressure would persuade Biden to drop out as soon as this weekend.
Biden's campaign however insisted that he would not quit. "He's staying in the race," deputy campaign chairman Quentin Fulks told a press conference on the sidelines of the Republican convention in Milwaukee.
"Our campaign is not working through any scenarios where President Biden is not the top of the ticket -- he is and will be the Democratic nominee."
California Senator Alex Padilla said Biden was "not skipping a beat." "I know having spoken to him personally he's committed to the campaign," he added.
Biden told reporters on Wednesday that he was "doing well." His Covid diagnosis however came at the worst possible time for his campaign, forcing him to cut short a trip to Las Vegas and isolate at his holiday home in Rehoboth, Delaware.
The split-screen with Donald Trump could not have been more stark, with Trump set to formally accept the Republican nomination in Milwaukee. US networks showed images of frail looking Biden gingerly descending the steps of Air Force One in Delaware, in a week when Trump is lauded by supporters each night at a packed party convention.
Former president Trump, who at 78 is just three years younger than Biden, is riding a wave of support from his party after surviving an assassination attempt on Saturday that left him with a bandaged ear.
The United States could now be approaching the climax of an extraordinary three weeks in politics, which started when Biden gave a disastrous performance during a televised debate with Trump.
Biden blamed jet lag and a cold, but the fact that America's commander-in-chief has now fallen ill for a second time just as fears grow about his fitness for the job has merely intensified the panic in Democratic ranks. The White House said on Wednesday that Biden was suffering mild symptoms including a runny nose and a cough and would "carry out the full duties of the office while in isolation."
White House spokesman Andrew Bates pushed back the previous night about the reports on Schumer and Jeffries, saying: "The President told both leaders he is the nominee of the party, he plans to win."