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A Democratic senator on Wednesday called on US 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 calls have been growing for 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.

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 TimesÌýon Wednesday just three weeks after co-hosting a huge fundraiser in Los Angeles that raised nearly $30 million for Biden.

"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," wrote Clooney.

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

Clooney said that Biden would lose the presidential election, and Democrats would also lose both chambers of Congress.

At the 15 June 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 performance in the 27 June television debate with Donald Trump.

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.

'Overwhelmingly negative'

But while top Democrats broadly but unenthusiastically backed him on Tuesday, Biden's efforts to stop the bleeding increasingly looked to be in vain just a day later.

Former House speaker Pelosi, 86, was lukewarm on his candidacy Wednesday, telling MSNBC "it's up to the president to decide if he is going to run."

"We're all encouraging him to make that decision because the time is running short," she said, despite Biden's repeated pledges last week to stay in the race.

Pelosi said Biden should delay any final decision until after NATO's 75th anniversary summit in Washington, which ends on Thursday with what will be a closely watched press conference by the president.

Fellow NATO leaders appeared to rally round Biden as he greeted them on arrival.

French President Emmanuel Macron, himself under pressure after calling an inconclusive snap election, gave the older man a hug and a firm handshake on the podium.

Biden, who gave a forceful speech pledging new air defenses for Ukraine as the summit opened on Tuesday, got through his opening remarks with only a couple of verbal stumbles.

But his every move from now until November will now be watched for evidence of age-related frailty or ill health.

Republicans also turned the screw, with the House Oversight Committee, which the party controls, issuing subpoenas for three top White House aides to testify about Biden's health, the Axios news outlet reported.