U.S. 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, U.S. 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 Nov. 5 election, in a tight race that will come down to a handful of battleground states.