Actor and donor George Clooney says Biden will lose the presidency, House and Senate if he doesn’t step aside.

United States President Joe Biden’s teetering re-election campaign has plunged deeper into crisis amid a fresh wave of pressure from allies concerned about his age and fitness, with actor George Clooney and several top Democrats joining calls for him to step aside.

In a withering essay published in The New York Times on Wednesday, Clooney said that he had personally witnessed the 81-year-old Democrat’s decline at a major fundraiser he co-hosted for the president last month and that 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,” Clooney wrote.

“We’re all so terrified by the prospect of a second Trump term that we’ve opted to ignore every warning sign.”

Clooney said Biden would not beat Republican challenger Donald Trump in November and that Democrats would also lose the House of Representatives and the Senate if he stayed on the ticket.

“This isn’t only my opinion; this is the opinion of every senator and Congress member and governor who I’ve spoken with in private. Every single one, irrespective of what he or she is saying publicly,” he wrote.

In an op-ed published in The Washington Post later on Wednesday, Democratic Senator Peter Welch, who represents Vermont, became the first member of the chamber to call on Biden to quit the race.

“We cannot unsee President Biden’s disastrous debate performance. We cannot ignore or dismiss the valid questions raised since that night,” Welch wrote.

“I understand why President Biden wants to run. He saved us from Donald Trump once and wants to do it again. But he needs to reassess whether he is the best candidate to do so. In my view, he is not.”

Also on Wednesday, two more House Democrats, Earl Blumenauer of Oregon and Pat Ryan of New York, and New York Lieutenant Governor Antonio Delgado called on Biden to withdraw.

Ten Democratic lawmakers have publicly called on Biden to drop out so far, while numerous others have raised concerns about his ability to defeat Trump.

Meanwhile, former House Speaker Nancy Pelosi declined to definitively back Biden’s candidacy in a television interview, saying he should make a decision about continuing in the race soon, even though the president has repeatedly insisted he will not drop out.

“I want him to do whatever he decides to do,” Pelosi told MSNBC. “We’re all encouraging him to make that decision because time is running short.”

Biden, who is hosting world leaders at the NATO summit in Washington this week, has repeatedly dismissed calls to step aside since a faltering debate performance against Trump that saw him trip over his words and lose his train of thought.

The veteran politician has insisted he is the best candidate to beat Trump and cast concerns about his candidacy as largely driven by the media and Democratic Party elites.

On Thursday, Biden, who has done far fewer media appearances than his predecessors, will hold a rare news conference in his latest effort to shore up support within his party and assuage doubts about his condition.

Biden’s re-election prospects have dimmed in recent weeks amid concerns over his age and widespread disaffection with the economy, with recent opinion polls suggesting Trump is in the lead both nationwide and in key battleground states.

On Tuesday, the non-partisan Cook Political Report switched Arizona, Georgia and Nevada – key swing states won by Biden in 2020 – from “toss-up” to “lean Republican”.

In a Reuters/Ipsos poll released last week, 59 percent of registered Democratic voters said Biden is too old for office, while one in three said he should step aside.

If Biden were to quit, it would cast the race into uncharted territory.

The US presidential primary season has already ended, although the party’s candidate will not be finalised until the Democratic National Convention next month.

Vice President Kamala Harris has been widely tipped as the most likely replacement for Biden if he were to withdraw, although polling shows her performing roughly the same as the president against Trump.