US President Joe Biden was reportedly coming closer to dropping out of the White House race Thursday as allies including Barack Obama were said to have expressed concerns that he can no longer win the 2024 election.

The 81-year-old Democrat was holed up at his beach house with Covid as multiple US media reports said he was considering the future of his reelection bid amid mounting opposition within his party.

The New York Times cited several people close to Biden as saying they believe he has begun to accept that he may lose in November to Republican rival Donald Trump and may have to drop out, with one quoted as saying: "Reality is setting in."

It quoted the person as saying it would not be a surprise if Biden soon endorsed Vice President Kamala Harris to replace him as the Democratic candidate.

Former president Obama meanwhile had told allies that Biden should "seriously consider the viability of his candidacy," the Washington Post reported. His office did not comment.

The Axios news outlet separately quoted party figures as saying that Biden could drop out as soon as this weekend, while broadcaster NBC quoted a person close to Biden as saying: "We're close to the end."

Meanwhile Jon Tester of Montana became the second Democratic senator to publicly call for Biden to quit the race, saying: "While I appreciate his commitment to public service and our country, I believe President Biden should not seek reelection to another term."

Biden's candidacy has been in peril for three weeks since a disastrous television debate against Trump rekindled fears about his age and fitness.

His campaign however insisted he was staying in. 

"He's running for reelection. Baseless conjecture from anonymous sources isn't a scoop," campaign official T.J. Ducklo said on X.

Harris made no comment on the crisis in a speech in the battleground state of North Carolina but praised the president, saying the "contrast between Joe Biden and Donald Trump, it's like night and day."

She also lashed out Trump's new running mate J.D. Vance, saying November's vote was the "most existential, consequential and important election of our lifetime."

Covid symptoms

While rival Trump prepares for his star turn at the Republican National Convention in Milwaukee later Thursday, Biden finds himself in both personal and political isolation.

His personal doctor, Kevin O'Connor, said Biden was still experiencing mild Covid symptoms and was taking the drug Paxlovid, but that his vital signs remain normal.

"He will continue to conduct the business of the American people," O'Connor said in a letter released by the White House.

His Covid diagnosis 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 Beach, Delaware.

Biden told reporters on Wednesday that he was "doing well" but was later seen looking frail as he slowly descended the steps of Air Force One.

The drumbeat of Democrats coming out against Biden is growing, with 20 members of the House of Representatives having openly called on him to quit.

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 and the New York Times reported.

White House National Security spokesman John Kirby meanwhile said Biden was being "kept up to speed" by his team despite the political turmoil and his illness.

However the split-screen with Donald Trump could not be more stark, with Trump set to formally accept the Republican nomination in Milwaukee as Biden hunkers down.

Republicans find themselves newly unified after former president Trump, who at 78 is just three years younger than Biden, survived an assassination attempt on Saturday that left him with a bandaged ear.

by Chris Kleponis with Danny Kemp in Washington (AFP)