President Donald Trump and his challenger Joe Biden have fiercely clashed in one of the most chaotic and bitter White House debates in years.
Mr Trump frequently interrupted, prompting Mr Biden to tell him to "shut up" as the two fought over the pandemic, healthcare and the economy.
The US president was challenged over white supremacist support and refused to condemn a specific far-right group.
Opinion polls suggest Mr Biden has a steady single-digit lead over Mr Trump.
But with 35 days until election day, surveys from several important states show a closer contest.
Polls also suggest one in 10 Americans have yet to make up their mind how to vote. But analysts said Tuesday night's debate - the first of three - probably would not make much difference.
Overall, the 90-minute debate in Cleveland, Ohio, was light on serious policy discussion. Both candidates talked over each other but Mr Trump cut in some 73 times, according to a count by CBS News.
The tenor became clear early on as the two candidates sparred over healthcare. Hectoring from Mr Trump saw Mr Biden call the president a "clown".
As they moved on to the Supreme Court, the rancour continued, with Mr Biden refusing to answer when asked if he would try to expand the number of judges.
"Will you shut up, man?" the Democratic candidate snapped at Mr Trump, later adding: "Keep yapping, man."
Mr Trump, the Republican candidate seeking a second term, responded: "The people understand, Joe. Forty seven years [in politics], you've done nothing. They understand."
Who won the Trump-Biden clash?
He mostly met that standard, although it was at least in part because Donald Trump, by his constant hectoring and interruptions, seldom gave the former vice-president a chance to say something truly damaging to his own cause.
Twitter Trump - the unconventional, bombastic, insulting and rumour-mongering aspect of this president - was on full display throughout the hour-and-a-half event. Unfortunately for the president, many Americans, even his own supporters, find his social media persona one of his more unattractive attributes.
Trump needed this debate to shake up a race that is tilting against him - and which has been remarkably stable, through economic, health and social adversity.
Nothing about this free-for-all seems likely to alter the dynamics of this contest or change the minds of the one in 10 American voters who say they are still undecided (although perhaps they'll resolve never to watch another one of these).