Washington DC - Kamala Harris on Saturday challenged Donald Trump to another debate in the lead-up to the US presidential election, but the Republican snubbed the offer, saying it was "too late."

Earlier in the day, Harris's campaign said she had accepted an invitation from broadcaster CNN to participate in a debate on October 23.

It would have been their second debate, after a September 10 encounter she was widely considered to have won.

"The American people deserve another opportunity to see Vice President Kamala Harris and Donald Trump debate before they cast their ballots," her campaign chair Jen O'Malley Dillon said in a statement.

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But Trump, speaking at a campaign rally in the battleground state of North Carolina, said he would like to debate – calling it "good entertainment value" – but that the start of early voting in some states had taken the air out of the idea.

"It's just too late, voting has already started," he said.

He added, to a large and enthusiastic crowd of supporters, that while CNN had been "very fair" when he debated President Joe Biden in June, "they won't be fair again" after criticism for the handling of the first debate.

Vice President Harris replaced her boss at the top of the Democratic ticket after the 81-year-old Biden's disastrous performance against Trump.

His exit from the race left Trump (78) now the oldest presidential nominee against a much younger Harris (59.).