Democratic presidential candidate Joe Biden said he worries President Donald Trump will try to "steal" the November election but he is confident soldiers would escort Trump from the White House if he loses and does not recognize the result.

"It's my greatest concern, my single greatest concern: This president is going to try to steal this election," Biden said in an interview broadcast late on Wednesday on Comedy Central's The Daily Show.

Biden did not specify how he thought Trump, a Republican, might try to steal the election. But the former vice president cited Trump's opposition to mail-in voting and said Democrats would have lawyers present at voting locations across the country to look out for Republican efforts to suppress the vote.

Trump has repeatedly attacked voting by mail, proclaiming without evidence that the expected increase in mail-in voting would lead to widespread fraud in the November 3 contest.

Biden said recent comments by former senior military officials criticising Trump's response to nationwide protests over police brutality made him confident the US military would intervene if Trump refused to accept the election results.

"I'm absolutely convinced they will escort him from the White House with great dispatch," Biden said.

Biden has often said publicly he worries Trump would try to cheat. But his rhetoric has escalated recently as most national polls show the former vice president leading the president, while Trump has stepped up criticism of voting by mail.

Trump's re-election campaign did not immediately respond to a request for comment. Trump and his Republican allies say mail voting is prone to fraud and favours Democrats, even as numerous independent studies have found little evidence of either claim.

During the 2016 presidential election campaign, Trump refused to say during a final debate with Democratic candidate Hillary Clinton that he would concede if Clinton won.

Election experts and officials expect a surge in mail-in voting this November due to concerns over the coronavirus pandemic and warn the process could be marred by chaos of the type already seen in primary elections held in states during the novel coronavirus outbreak.

A large number of mail ballots not delivered in time to be cast or counted could lead to legal challenges over election results. Counting mail ballots also take more time because a voter's identity must first be validated, raising the prospect that the election outcome will not be known well past Election Day, experts say.