New York, New York - President Joe Biden attended a triple-header of campaign fundraisers Saturday, seeking to reassure high-dollar donors he can still win reelection in November despite a debate performance that sparked panic among many Democrats.

Accompanying him at the fundraisers in New York and New Jersey was First Lady Jill Biden, who has fiercely defended her 81-year-old husband amid calls for him to step aside.

"Joe isn't just the right person for the job – he's the only person for the job," she told one gathering, which featured a-list actors Sarah Jessica Parker and Matthew Broderick among the co-hosts.

The president is facing a wave of doubts following Thursday night's debate against Republican rival Donald Trump, after he frequently stumbled over his words and lost his train of thought – exacerbating fears about his age and mental acuity.

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Many political commentators called for Biden to stand down following the debate, including The New York Times editorial board.

The Washington Post's editorial board, meanwhile, urged him to do some soul-searching over the weekend after his "calamitous" debate performance raised "legitimate questions about whether he's up for another four years in the world's toughest job."

No high-ranking elected Democrat has yet joined the call, and former presidents Barack Obama and Bill Clinton both publicly reiterated their backing on Friday.

Biden attempted to tamp down the nay-saying with a fiery campaign speech Friday in North Carolina in which he pledged to keep fighting.

"I don't walk as easy as I used to. I don't speak as smoothly as I used to. I don't debate as well as I used to," Biden admitted to supporters. "But I know how to tell the truth. I know how to do this job," he said to huge cheers, vowing "when you get knocked down, you get back up."

Biden's campaign has accepted that the debate did not go how they had hoped, but insists the neck-and-neck race against Trump has not been significantly altered.

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Jennifer O'Malley Dillon, chair of Biden's campaign, said in a public memo Saturday that internal post-debate polling showed that "voters' opinions were not changed." She said there had actually been a surge of support during and following the debate, with $27 million raised by Friday evening.

"I didn't have a great night, but neither did Trump," Biden told one of the gatherings Saturday. "I promise you we're gonna win this election."

Later, he and the first lady flew to neighboring New Jersey to attend a fundraiser with the state's Democratic governor, Phil Murphy.

"I understand the concern after the debate," Biden told the gathering. "I get it. I didn't have a great night, but I'm going to be fighting harder," he pledged.

Murphy told the crowd that Biden is "on fire and we are all with you 1,000%."

Biden was later set to travel with his family to the Camp David presidential retreat, where he had spent the previous week preparing for the debate with close aides.