WASHINGTON: US President Joe Biden said on Friday he is confident the upcoming presidential election will be fairly conducted, but he warned that Republican candidate Donald Trump and his running mate could refuse to accept the outcome.

"I'm confident it will be free and fair. I don't know whether it will be peaceful. The things that Trump has said, and the things that he said last time out, when he didn't like the outcome of the election, were very dangerous," Biden said.

Biden said it was notable that Trump's running mate, US Senator JD Vance, would not confirm during this week's vice presidential debate that he would accept the outcome of the vote next month.

Trump is running against Democratic Vice President Kamala Harris in the November 5 election, in a tight race that will come down to a handful of battleground states.

Trump, visiting a Hurricane Helene response centre in Evans, Georgia, on Friday, said he had not heard what Biden had said.

"I only can hope that it's going to be free and fair," he told reporters. "I think in this state it will be, and I hope in every state it will be."

US prosecutors said this week that Trump had acted outside the scope of his duties as president when he pressured state officials and then-Vice President Mike Pence to try to overturn his 2020 election defeat.

Their 165-page filing is likely the last opportunity for prosecutors to detail their case against Trump before the presidential election.

Asked by moderators during the vice presidential debate on Tuesday, "Would you again seek to challenge this year's election results, even if every governor certifies the results?" Vance responded by saying that he was "focused on the future." Vance then said, "Look, what Trump has said is that there were problems in 2020."