Biden supporters had hoped Thursday night's debate would dispel worries that the 81-year-old was too old for another term, but the 90-minute clash appeared to do the opposite. 

Biden and Trump, 78, both have faced concerns about their age and fitness in the run-up to the Nov. 5 election, but they have weighed more heavily on Biden. Now, with less than five months before the US goes to the polls, some Democratic strategists and fundraisers are privately asking whether the president must be replaced at the top of the party ticket.

His voice hoarse from what White House officials told reporters was a cold, Biden hurried through some of his talking points on the debate stage, stumbled over some answers and trailed off during others. 

About halfway through the debate, a Democratic strategist who worked on Biden’s 2020 campaign called it a "disaster."

"There is no way to spin this. His performance was disqualifying," a top Democratic fundraiser who spoke on condition of anonymity told Reuters after the event.

He predicted there would be a push for a so-called brokered convention when the Democratic National Convention is held in August to nominate Biden - referring to the process when a convention fails to choose a party nominee in the first round of voting, and delegates negotiate who should be named instead. 

"I expect fundraising to dry up. Money follows enthusiasm," the fundraiser said. "How can anyone with a straight face say 'Donate to elect Joe'" 

Early in the debate, Biden paused as he was making a point about Medicare and tax reform and seemed to lose his train of thought. 

Tax reform would create money to help "strengthen our healthcare system, making sure that we're able to make every single solitary person eligible for what I was able to do with the, with the COVID, excuse me, with dealing with everything we had to do with," Biden said, pausing. "We finally beat Medicare."

Trump jabbed Biden for being incoherent, saying at one point: "I really don't know what he said at the end of that sentence. I don't think he knows what he said."

Prominent Democratic lawmakers are worried, a Congressional aide who works for a Democratic senator said. She noted her boss is "sitting with other frontline members and panicking. Let the Democratic bedwetting begin."

David Axelrod, a top strategist for former President Barack Obama, told CNN: "There are gonna be discussions on if he should continue." 

Biden campaign officials sought to put on a brave face, saying voters should focus on the substance of the debate. 

Campaign Chair Jen O’Malley Dillon contrasted Biden's performance with Trump, saying he delivered "a positive and winning vision for the future of America – one in which every American has a fair shot at the American dream." 

Ahead of the debate, Biden confined himself to nearly a week of "debate camp" with top advisers at the Camp David presidential retreat in the mountains of western Maryland, an indication of how important his campaign considered Thursday night. It didn't reflect on his performance, critics said.

While Biden struggled, Trump rattled off a series of well-worn falsehoods like migrants carrying out a crime wave and Democrats supporting infanticide. 

"Trump is Trump, every word out of his mouth is bullshit. But Biden sounds old. And lost. And that’s going to matter more than anything. So far, this is an absolute nightmare for Biden," Joe Walsh, a former 2020 Republican presidential candidate who has been critical of Trump, said on X.