Trump criticised Democratic governors who have imposed restrictions that aim to slow the virus's spread, and said Biden would prohibit Americans from gathering for holidays or other special occasions if elected. Many of those who went to see him did not wear masks.

"You've got to open up your state and you've got to do it fast!" Trump said on Friday at a rally in Green Bay, Wisconsin, with just four days to go before the election.

At a later event in Rochester, Minnesota, he greeted hundreds of supporters who were locked out of a rally that was limited to 250 participants by state authorities.

Trump also trained his ire on the US medical system, falsely saying doctors are somehow incentivised to drive up the death count.

"Our doctors get more money if someone dies from COVID," he said in Waterford Township, Michigan.

Biden accused Trump of "giving up" in the fight against the virus and said he should not attack medical personnel who were treating its victims.

"Unlike Donald Trump, we will not surrender to this virus," he said at a rally in St Paul, Minnesota.

The coronavirus pandemic, which has killed nearly 230,000 people in the United States and cost millions more their jobs, has dominated the final days of the campaign.

A record surge of cases is pushing hospitals to the brink of capacity. The news pushed Wall Street to its worst week since March, undercutting one of Trump's main arguments for re-election.

Trump, who recovered from COVID-19 weeks ago, has played down the health crisis for months, telling supporters in recent weeks that the country is "turning the corner" even as cases surge. Biden has warned of a "dark winter" ahead and promised a renewed effort to contain the virus.

Biden leads Trump 52 per cent to 42 per cent in Reuters/IPSOS national opinion polling, partly because of widespread disapproval of his handling of the pandemic. Opinion polls show a closer contest in the most competitive states that will decide the election.

The focus on the upper midwest underlined the region's importance in the race. Michigan and Wisconsin were two of the three historically Democratic industrial states, along with Pennsylvania, that narrowly voted for the Republican Trump in 2016, delivering him an upset victory.

Biden leads Trump by nine percentage points in Michigan and Wisconsin and five points in Pennsylvania, according to Reuters/IPSOS polling.

The pandemic and an extraordinary level of enthusiasm have prompted Americans to vote early in unprecedented numbers.

More than 86 million votes have been cast either by mail or in person, about 63 per cent of the total number of votes in the entire 2016 election, according to the US Elections Project at the University of Florida.

In Texas, a traditionally Republican state where polls show Biden and Trump close, more than nine million people have cast ballots, eclipsing total turnout from 2016.

Early voting data shows far more Democrats have voted by mail, while Republicans are expected to turn out in greater numbers on Tuesday.

Last-minute legal battles have added to the uncertainty.

On Thursday, a federal appeals court said Minnesota's plan to count ballots that arrive after election day was illegal. That ruling came after the Supreme Court allowed North Carolina and Pennsylvania to keep similar deadlines in place.

Separately, a federal judge in Washington ordered the US Postal Service to take extra steps to ensure speedy delivery of mail ballots in areas where service has been slow.

Australian Associated Press