Donald Trump and rival Joe Biden have clashed over the president's record on the coronavirus pandemic, healthcare and the economy in a chaotic and bad-tempered first debate marked by insults and Trump's repeated interruptions.
The president bulldozed his way through the 90-minute debate, trying to goad Biden, claiming his Democratic party was trying to steal the November election and declining to condemn white supremacist groups.
Moderator Chris Wallace never established control, with Trump repeatedly ignoring his calls to let Biden speak.
The two White House contenders talked over each other and lobbed insults in a breathtaking political brawl that made it hard for either to make a point.
At one stage, an exasperated Biden said after Trump's repeated interruptions: "Will you shut up, man? This is so unpresidential."
For Trump, 74, Tuesday's debate represented one of the few remaining chances to change the trajectory of a race that most opinion polls show him losing, as the majority of Americans disapprove of his handling of the coronavirus pandemic and protests over racial injustice.
Biden, 77, has held a consistent lead over Trump in national opinion polls, although surveys in the battleground states that will decide the election show a closer contest.
Trump, asked if he was willing to condemn white supremacists and tell them to stand down, initially said he would be willing to do anything for peace but then said most of the violence was from the left wing.
When Biden mentioned the Proud Boys, an organisation that describes itself as a club of "Western chauvinists", Trump said: "Proud Boys, stand back and stand by."
He also repeated his unfounded complaints that mail-in ballots would lead to widespread fraud in the election and declined to commit to accepting the results of the election or commit to a peaceful transfer of power if he loses the election.
"If I see tens of thousands of ballots being manipulated, I can't go along with that," he said. "This is going to be a fraud like you've never seen."
Biden was sharply critical of Trump's record on the pandemic which has killed more than 200,000 Americans, accusing him of failing to protect Americans because he was more concerned about the economy.
"He panicked or he looked at the stock market," Biden said of Trump, who has pushed for states to reopen their economies and played down the threat of the pandemic.
"A lot of people died and a lot more are going to die unless he gets a lot smarter, a lot quicker," Biden said.
Trump objected to Biden using the word "smart" and defended his approach on the pandemic. "We have done a great job."
Trump defended his effort to swiftly fill a US Supreme Court seat, saying he had the right despite Democratic objections.
"I will tell you very simply we won the election, elections have consequences. We have the Senate and we have the White House and we have a phenomenal nominee respected by all," Trump said in defence of his nominee, conservative Judge Amy Coney Barrett.
Biden said the seat of the late liberal Justice Ruth Bader Ginsburg should be filled after the election, when it was clear who the president would be.
He said a more conservative Supreme Court would endanger the Affordable Care Act known as Obamacare.
Hours before the debate, Biden released his 2019 tax returns and his campaign called on Trump, who has come under fire for not releasing his returns, to do the same.
Biden took the step two days after the New York Times reported Trump paid just $750 ($A1055) in federal income taxes in 2016 and 2017 - and none in 10 of the previous 15 years - following years of reporting steep losses from business enterprises.
Trump had long sought to keep his personal financial records secret.