A raspy and sometimes halting Biden tried repeatedly to confront Trump, as his Republican rival countered the president's criticism by leaning into falsehoods about the economy, illegal immigration and his role in the January 6 2021 Capitol insurrection.
Biden's uneven performance, particularly early in the debate, crystallised the concerns of many Americans that, at 81, he is too old to serve as president.
There were renewed calls for the Democrat to consider stepping aside as the party's nominee before the November election as members of his party fear 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 drew the most attention afterwards.Â
Trump's allies immediately declared victory, while prominent Democrats publicly questioned whether Biden could move forward.
"I think the panic had set in," David Axelrod, a longtime advisor to former US president Barack Obama, said 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."
Another Democratic strategist Said Van Jones, also on CNN, said: "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 had 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."
