The Biden campaign had hoped that a strong debate would quell concerns among voters that the 81-year-old Democratic incumbent is too old to serve a second four-year term.
Instead, a hoarse-sounding Biden stumbled over his words at times, especially in the early stages of the debate.
One Biden donor, who spoke on condition of anonymity, called his performance "disqualifying" and predicted some Democrats would revisit calls for Biden to step aside in favour of another candidate before the party's national convention in August.
Panicking Democrats exchanged messages wondering whether Biden would consider stepping down.
One donor branded President Joe Biden's performance against Donald Trump "disqualifying". (AP PHOTO)
One senior Democratic strategist said it would be unprecedented for an incumbent to pull out this late in the election cycle, but there would be calls for Biden to do just that.
Interviews with undecided voters confirmed it was a bad night for Biden. They described his showing as feeble, embarrassing and difficult to watch.
Two White House officials said mid-debate that Biden had a cold, but that only further reflected his aides' anxiety about his performance.
Vice-President Kamala Harris conceded that Biden had a "slow start" but argued that his record over three-and-a-half years as president outweighed one 90-minute event.
California Governor Gavin Newsom, who could be a leading Democratic alternative if Biden stepped aside, dismissed the notion that Biden could be replaced.
Trump, 78, has also faced questions about his fitness for office, given his conviction in May in New York for covering up hush-money payments to a porn star, his efforts to overturn the 2020 election and his chaotic term in office.
But despite a litany of falsehoods from Trump during the debate, the focus in the aftermath was squarely on the incumbent.
Neither candidate is popular, and polls show many Americans are dissatisfied with their options.
Some voters described Joe Biden's showing as feeble, embarrassing and difficult to watch. (AP PHOTO)
The debate on Thursday at CNN headquarters in Atlanta took place more than four months before the November 5 election.
That could mitigate the damage for Biden, as news events generate fresh headlines.
Trump, for instance, is scheduled to be sentenced in New York on July 11, just days before his party convenes to formally nominate him.
He still faces three other criminal indictments, though none appears likely to reach trial before November.
As Biden stopped for food at a Waffle House in the wee hours of Friday, he told reporters that "I think we did well".
Asked if he was concerned about his performance, he said, "No. It's hard to debate a liar."
With polls showing the race in a dead heat, the candidates clashed over the economy, abortion, immigration and the wars in Ukraine and Gaza in a debate that included some deeply personal attacks.
Trump unleashed numerous familiar false claims, including that the 2020 election was fraudulent, that Democrats support infanticide and that migrants have carried out a wave of violent crime.
Joe Biden told reporters at an eatery after the debate that he thought he did well. (AP PHOTO)
He defended his supporters arrested for storming the US Capitol on January 6, 2021, refused to say whether he would accept the election results and suggested he might prosecute Biden if he wins.
But Biden struggled to fact-check his predecessor, and CNN moderators Jake Tapper and Dana Bash did not correct Trump on air.
Biden landed some blows. He called Trump a felon and noted that the majority of Trump's former cabinet have declined to endorse his campaign.
"This guy has no sense of American democracy," Biden said during a segment on the January 6 attack.
Biden also blamed Trump for enabling the elimination of a nationwide right to abortion by appointing conservatives to the US Supreme Court, an issue that has bedevilled Republicans since 2022.
Biden called Trump a "whiner" and a "child" who, he said, cheated on his wife with a porn star; Trump said Biden was a "disaster" and a "Manchurian candidate" who favoured China over the US.
At one point, the debate devolved into a fight over which man had a better golf game.
The second and final debate is scheduled for September.
