— Donald Trump vowed to again defy the polls as he sprinted through five swing states on Sunday, while his opponent Joe Biden urged supporters to "take back our democracy" by voting in two days.

The last-minute scramble came as polls showed Biden maintaining his overall lead — but with some slight tightening in key states including Pennsylvania, where he leads by four points, and Florida, now a tossup, according to a RealClearPolitics average of polls.

With Americans galvanised by the stakes, the election has already mobilised a record number of early voters.

"We're now leading," Trump insisted before a raucous rally of supporters in Washington Township, Michigan.

"Look, we're leading in Florida. We're leading in Georgia... They say it's a very close race in Texas. I don't think so. They did that four years ago and I won in a landslide."

Snow flurries fell on Trump and the crowd as he shivered and joked repeatedly about the brisk winds and freezing conditions.

He warned, in a state long dependent on manufacturing, that Biden had "spent 47 years outsourcing your jobs, opening your borders and sacrificing American blood and treasure in endless foreign wars".

Biden and his wife Jill began the day attending Mass at their Catholic church near their home in Wilmington, Delaware.

The former vice president spent the rest of the day in a neighbouring state that is vital to both men's prospects: Pennsylvania.

At a drive-in rally in Philadelphia, Biden said: "In two days, we can put an end to a presidency that has divided this nation."

"It's time to stand up, take back our democracy," he said.

"We can do this. We're better than this. We're so much better."

'Almost criminal' 

He also continued to hammer Trump over his handling of the coronavirus pandemic — which he called "almost criminal" in an earlier speech to supporters — following the country's worst week for new cases ever, with more than 1,000 Americans dying daily.

Yet, the president has continued to shrug off the seriousness of COVID-19 — going so far as to accuse doctors of inflating virus death tolls for profit.

At his third rally of the day in Hickory, North Carolina, Trump called for businesses and schools to reopen and touted signs of a recovering economy — though economists say underlying factors do not bode well.

His extraordinary conflict with doctor Anthony Fauci, the widely respected government expert on infectious diseases, also continued.

In an interview in The Washington Post published on Saturday, Fauci said bluntly that without "an abrupt change” in the country’s public health practices, Americans face "a whole lot of hurt ahead”.

But he praised the Biden campaign which — in contrast to Trump’s mass rallies — follows health guidance in its public events.

Fauci’s remarks drew a sharp rebuke from a White House spokesman, Judd Deere, who called it "unacceptable” for Fauci "to choose three days before an election to play politics”.

Fears of tensions on election night and afterward, with vote counting expected to continue due to the large number of mail-in ballots, were further stoked by a report that Trump could declare victory prematurely.

The Axios news site reported that Trump has told confidants he would declare victory on Tuesday night if it looks like he is ahead.

Trump called it a "false report”, but also repeated his argument that "I don’t think it’s fair that we have to wait for a long period of time after the election”.

He has repeatedly claimed, without evidence, that mail-in ballots are open to fraud, and warned on Saturday of "bedlam in our country” if no clear winner emerges quickly.

When asked about the report, Biden said: "The president is not going to steal this election.”

Boarding up 

In another sign of how unusual and tense the race has become, Biden also denounced the alleged harassment of one of his campaign buses by Trump supporters on a Texas highway — an incident the FBI confirmed it is investigating.

The election takes place in a deeply divided country, with feelings so raw that gun sales have surged in some areas.

Businesses in some cities, including Washington, are protectively boarding windows, and police are preparing for the possibility of violence.

Trump started an exhausting Sunday schedule with successive rallies in Michigan, Iowa, North Carolina and Georgia before an unusual 11:00pm rally in Florida.

On Monday, both Trump and Biden will campaign again in Pennsylvania, which Trump won in 2016 by less than 1 percentage point. The president will later return to Michigan, and Biden reportedly is adding a last-minute stop in Cleveland, Ohio.

Biden’s former boss Barack Obama, who has excoriated Trump in his appearances on the campaign trail, will hold an election eve rally in Miami, the campaign announced.

A record 93 million early votes have already been cast, according to the non-partisan US Elections Project.

In Trump’s 2016 victory, he was able to flip the crucial states of Wisconsin, Pennsylvania and, by an especially narrow margin, Michigan.

But polls show Biden, despite his more cautious and reserved campaign style, ahead there while also pushing Trump onto the back foot in traditionally conservative battlegrounds like Georgia and Texas.