US President Joe Biden suffered a stunning blow on Wednesday when actor and leading Democratic supporter George Clooney urged him to drop his reelection bid, while party heavyweight Nancy Pelosi declined to back his candidacy.

"I love Joe Biden," Clooney, who hosted a star-studded fundraiser with Biden just last month, wrote in the New York Times. "But the one battle he cannot win is the fight against time.

"It's devastating to say it, but the Joe Biden I was with three weeks ago at the fundraiser was not the Joe 'big F-ing deal' Biden of 2010.

"He wasn't even the Joe Biden of 2020. He was the same man we all witnessed at the debate."

At the fundraiser in Los Angeles co-hosted by Clooney and fellow movie star Julia Roberts, Biden appeared tired as he took to the stage alongside former president Barack Obama.

He had flown straight to California from the G7 summit in Italy and has since blamed jetlag, and a cold, for his disastrous performance in a June 27 television debate with Donald Trump.

Biden's bid for a second term in the White House has been under severe pressure since the debate, in which he slurred his words and stood with mouth agape while Trump talked.

While top Democrats broadly but unenthusiastically backed him on Tuesday, Biden's efforts to turn the page took a leap backwards with the interventions of Pelosi and Clooney.

Former House speaker Pelosi, 86, said time was running out for Biden to decide whether to end his candidacy, ignoring Biden's repeated insistence that he is committed to run.

"It's up to the president to decide if he is going to run," she told MSNBC.

"We're all encouraging him to make that decision because the time is running short."

Pelosi said Biden should however delay any final decision until after Nato's 75th anniversary summit in Washington, which the US president is hosting this week.

On the way to the summit on Wednesday, Biden dropped in on US union leaders in a further bid to shore up backing for his reelection bid.

Biden's appearances at the Nato gathering are being closely watched as he tries to reassure allies of his continued ability to lead as the alliance backs Ukraine's fight against Russia.

Seven House Democrats have openly called on Biden to not seek reelection. A handful of senators have also expressed concerns.

Late on Tuesday, Colorado's Michael Bennet became the first Senate Democrat to publicly turn on the president, saying Biden would lose if he stayed on the ballot – and perhaps cause congressional Democrats to lose as well.

"Donald Trump is on track, I think, to win this election, and maybe win it by a landslide and take with it the Senate and the House," Bennet told CNN, adding he believes the White House "has done nothing" to demonstrate they have a plan to win in November.

Republican challenger Trump, 78, is meanwhile back on the warpath after a long interval of silence following the June 27 debate, accusing Biden of hiding secrets about his health.

"It's the biggest cover-up in political history," Trump thundered at a rally in Florida on Tuesday.

The former president dared his successor to another debate without moderators and challenged him to a round of golf. (AFP)