A Democratic senator on Wednesday (Jul 11) called on United States President Joe Biden to ditch his reelection bid, capping a brutal day that saw mounting pressure from high-profile actor and donor George Clooney and party heavyweight Nancy Pelosi.

As the 81-year-old tried to show his leadership credentials at a NATO summit in Washington, domestic pressure mounted on Biden to quit following his disastrous debate performance against Republican challenger Donald Trump.

At least eight House Democrats have openly called on Biden to not seek reelection, but Peter Welch became the first in the Senate to explicitly do so as concerns rise over his age and fitness.

"For the good of the country, I'm calling on President Biden to withdraw from the race," the Vermont senator said in an opinion piece in the Washington Post.

News outlet Axios reported on Wednesday that Senate Majority Leader Chuck Schumer had also privately signaled to donors that he was open to replacing Biden on the Democratic ticket.

However, in a statement shared by his office on Wednesday evening, Schumer said: "I support President Biden and remain committed to ensuring Donald Trump is defeated in November."

Biden has been trying to stem a growing tide of Democrats saying that he cannot win in November, but a steady drip of public revolt is squashing efforts to turn the page on the crisis.

Hollywood star Clooney penned a devastating editorial in the New York Times just three weeks after co-hosting a huge fundraiser in Los Angeles that raised nearly US$30 million for Biden.

"I consider him a friend, and I believe in him ... But the one battle he cannot win is the fight against time," Clooney wrote.

"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 added.

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

The Democrats risk losing control of the Senate and won't gain a majority in the House of Representatives, said the Oscar-winning actor.

"This isn't only my opinion; this is the opinion of every senator and Congress member and governor that I've spoken with in private," Clooney wrote.

"This is about age. Nothing more. But also nothing that can be reversed."

At the Jun 15 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 the Jun 27 television debate with Donald Trump.

Fellow actor Michael Douglas followed up on Wednesday, saying he was "deeply, deeply" concerned about Biden's prospects.

In response to Clooney's editorial, the Biden campaign pointed to the president's statements on Monday saying he was "committed" to running again in November.

Media-shy for much of his presidency, Biden will also give a fresh interview, to broadcaster NBC on Monday, as he seeks to convince voters.