Clooney added to growing pressure from within the party in an opinion piece for The New York Times, warning: “We are not going to win in November with this President.”

“This is about age. Nothing more,” he wrote in the article titled “I love Joe Biden. But we need a new nominee”.

The Ocean’s 11 actor said 81-year-old Biden could not win 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,” Clooney wrote.

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

“Was he tired? Yes. A cold? Maybe. But our party leaders need to stop telling us that 51 million people didn’t see what we just saw.”

The A-lister said Democrats were “so terrified” by the prospect of a second Donald Trump presidency that they were ignoring all the warning signs.

“As Democrats, we collectively hold our breath or turn down the volume whenever we see the President, who we respect, walk off Air Force One or walk back to a mic to answer an unscripted question,” Clooney wrote.

He said it wasn’t just his opinion, but that of “every senator and congress member and governor that I’ve spoken with in private. Every single one, irrespective of what he or she is saying publicly”.

Last month, Clooney co-hosted an event with fellow actor and friend Julia Roberts, which raised more than $US30 million ($44 million) for Biden’s re-election campaign – the largest Democratic fundraiser in history.

Clooney has not endorsed a replacement candidate, but said it could be an exciting time for democracy that could enliven voters.

“We can easily foresee a group of several strong Democrats stepping forward to stand and tell us why they’re best qualified to lead this country,” he wrote.

“Joe Biden is a hero; he saved democracy in 2020. We need him to do it again in 2024.”

Pressure piles on

Hours earlier, one of the most powerful Democrats on Capitol Hill, Nancy Pelosi, also warned that “time is running short”.

But the former House speaker encouraged her colleagues to refrain from airing concerns while Biden hosted NATO leaders in Washington DC this week.

“I’ve said to everyone: Let’s just hold off. Whatever you’re thinking, either tell somebody privately but you don’t have to put that out on the table until we see how we go this week,” she said.

She declined to say definitively that she wanted Biden to run.

“I want him to do whatever he decides to do,” she said.

“We’re all encouraging him to make that decision because time is running short.”

Democrats in Congress remain deeply divided over whether to fall in line behind Biden or to urge him to step aside because of persistent questions about his health and acuity.

Biden has said he is fit to serve but understands the questions.

Some have expressed concern that Biden remaining at the top of the ticket could cost the Democrats the White House and both houses of Congress in November’s election.

But public defections remain a small segment of the 213 Democratic-aligned House members, and the party’s leadership continues to back Biden publicly.

No members of the Senate have publicly said Biden should stand aside. However, Colorado Senator Michael Bennet said on Tuesday he did not believe Biden could beat Trump.

House and Senate Democrats met privately on Tuesday with tensions running high.

The conversation was “dour” and “sad” in the House, legislators said, as they discussed their party leader who emphatically refuses to step aside and implored them in a sharply worded letter to refocus from him to the threat posed by Republican Trump.

In the Senate, where Biden spent a storied career, they said even less.

Late in the day, a seventh House Democrat, Mikie Sherrill of New Jersey, publicly said Biden should not run for re-election.

A military veteran, Sherrill said with Trump running for the White House, “the stakes are too high — and the threat is too real — to stay silent”.

“I realise this is hard, but we have done hard things in pursuit of democracy since the founding of this nation,” she said.

“It is time to do so again.”

– with AAP