The high number of early voters, about 65 per cent of the total turnout in 2016, reflects intense interest in the contest, with three days of campaigning left.

Concerns about exposure to the coronavirus at busy election day voting places on Tuesday have also pushed up the numbers of people voting by mail or at early in-person polling sites.

The Republican president is spending the closing days of his re-election campaign criticising public officials and medical professionals who are trying to combat the coronavirus pandemic even as it surges back across the United States.

Opinion polls show Trump trailing former vice-president Biden nationally but with a closer contest in the most competitive states that will decide the election. Voters say the coronavirus is their top concern.

Trump has repeatedly claimed without evidence that mail-in ballots are susceptible to fraud and has more recently argued that only the results available on election night should count. In a flurry of legal motions, his campaign has sought to restrict absentee balloting.

"I don't care how hard Donald Trump tries. There's nothing - let me say that again - there's nothing that he can do to stop the people of this nation from voting in overwhelming numbers and taking back this democracy," Biden said on Saturday at a rally in Flint, Michigan, where he was joined by former president Barack Obama for their first 2020 campaign event together.

Trump held four rallies on Saturday in the battleground state of Pennsylvania, where the campaigns are seeking to win over undecided voters in areas such as the suburbs of Philadelphia and the "Rust Belt" west of the state.

"If we win Pennsylvania, it's over," Trump told a large rally in Reading before moving to another big gathering in Butler.

Officials in several states, including Pennsylvania and Wisconsin, say it could take several days to count all the mail ballots, possibly leading to days of uncertainty if the outcome hinges on those states.

A federal judge in Texas has scheduled an emergency hearing for Monday on whether Houston officials unlawfully allowed drive-through voting and should toss more than 100,000 votes in Democratic-leaning Harris County.

In Iowa, a new poll shows Trump has taken over the lead there. The Des Moines Register/Mediacom Iowa poll shows Trump now leads Biden by seven percentage points.

At a small, in-person rally in Newtown, Pennsylvania, Trump mocked his opponent for his criticism of the administration's record of fighting COVID-19, which has killed more people in the United States than in any other country.

"I watched Joe Biden speak yesterday. All he talks about is COVID, COVID. He's got nothing else to say. COVID, COVID," Trump told the crowd, some of whom did not wear masks.

He said the United States was "just weeks away" from mass distribution of a safe vaccine against COVID-19, which is pushing hospitals to capacity and killing up to 1000 people in the United States each day. Trump gave no details to back up his remarks about an imminent vaccine.

Biden has accused Trump of being a bully, criticised his lack of a strategy to control the pandemic, which has killed nearly 229,000 Americans; his efforts to repeal the Obamacare healthcare law; and his disregard for science on climate change.

He has offered his own made-in-America economic platform, a contrast with Trump's "America First" approach, saying he will get the wealthy to pay their fair share and make sure earnings are distributed more equitably.

At one Biden rally in Detroit on Saturday, social distancing broke down as supporters crowded toward the stage to hear Obama speak.

Australian Associated Press