It sparked a fresh round of calls for the Democrat to consider stepping aside as the party's nominee amid fears of a return of Trump to the White House.
Biden repeatedly tore into Trump in an apparent effort to provoke him, bringing up everything from the former president's recent felony conviction to his alleged insult of First World War veterans, to his weight.
Donald Trump's allies declared victory while prominent Democrats raised doubts about Joe Biden. (AP PHOTO)
The 78-year-old Trump declined to clearly state he would accept the results of the November election, four years after he promoted conspiracy theories about his loss that culminated in the January 6 insurrection, and repeatedly misstated the record from his time in office.
But Biden's delivery from the beginning of the debate on Thursday drew the most attention afterwards.
Trump's allies immediately declared victory while prominent Democrats publicly questioned whether Biden could move forward.
"I think the panic had set in," said David Axelrod, a long-time adviser to former president Barack Obama on CNN immediately after the debate about Biden's performance.
"And I think you're going to hear discussions that, I don't know will lead to anything, but there are going to be discussions about whether he should continue."
Said Van Jones, another Democratic strategist, on CNN: "He did not do well at all."
Rosemarie DeAngelis, a Democrat who watched the debate at a party in South Portland, Maine, said she felt Biden gave the right answers to Trump but "didn't have the spark that we needed tonight".
"That's going to be the challenge going forward. This is only June, this is the first, but can he sustain?" she said.
"That is going to be the challenge."
Vice-President Kamala Harris, speaking on CNN afterwards, sought to defend the president's performance while acknowledging the criticism.
"There was a slow start, but there was a strong finish," she said.
Asked about his performance in the debate, Biden told reporters early on Friday that "I think we did well", but said he has a "sore throat".
Pressed about Democratic concerns with his showing that he should consider stepping aside, Biden said: "No, it's hard to debate a liar."
