Donald Trump and Joe Biden square off with rallies in key Midwestern states on Friday as a resurgent coronavirus further highlights their sharp differences just four days before the US presidential election.

Even with the United States reaching another daily record in new Covid-19 infections on Thursday, US President Trump stuck to his strategy of downplaying its danger and calling for businesses to reopen.

He has also ramped up fear-mongering, warning of rampaging “socialists” and seeking to portray his Democratic challenger as intent on shutting the country down.

Biden has sought to persuade the few remaining undecided voters that he would provide a steady hand on the wheel and heal America's “soul”, calling Trump irresponsible.

The former vice president has held drive-in, socially distanced rallies, while Trump's gatherings have often seen attendees ignore such guidelines and eschew masks.

The two candidates plan to be in three Midwestern states on Friday, with both campaigning in Wisconsin and Minnesota. Trump will also travel to Michigan and Biden also plans to be in Iowa.

Trump's wins in Michigan, Wisconsin and Iowa helped propel him to victory in 2016.

But polling averages from the RealClearPolitics.com site show Biden up in all four of those states, ranging from 6.5 percentage points in Michigan to just one point in Iowa.

Still, Democrats and Republicans alike have repeatedly warned of the unreliability of polls, pointing to Trump's shock defeat of Hillary Clinton in 2016 despite surveys showing her to be the clear favorite.

Florida rallies

Trump and Biden are concentrating their efforts on battleground states that will decide the election in the days before the November 3 polls, and on Thursday both were in the crucial state of Florida.

Trump, 74, held another raucous rally in Tampa, telling the cheering crowd that coronavirus lockdowns under Biden would banish normal life.

“They will allow you nothing,” the Republican said. “We're never going to lock down again [...] We're open for business,” he said, telling supporters that his own recent bout with Covid-19 proved that it can be beaten.

“You know, the bottom line is you get better,” he said.

But the pandemic, which has already taken 228,000 American lives, has shown its resilience and is undergoing a long-predicted second wave.

On Thursday, more than 91,000 new US infections were recorded, according to a Johns Hopkins University tally, the highest 24-hour total since the pandemic began.

Trump however did receive good news on Thursday in his argument that he is best placed to bring about an economic recovery.

New figures showed an annual growth rate of 33.1 per cent in the third quarter — a jaw-dropping statistic that reflected the economy's rebound from such a low base.

Biden, addressing a socially distanced drive-in event in Broward County, reminded supporters that of all the states there are few as important as Florida in deciding the outcome of tight elections.

Rebuffing Trump's central charge, the 77-year-old emphasised his claim that he would bring responsible leadership after months of the White House downplaying the virus' danger.

“I'm not going to shut down the economy, I'm not going to shut down the country. I'm going to shut down the virus,” he said in Tampa.

While Trump mocks him for holding small campaign events, Biden said he was leading by example, instead of staging the president's “super-spreader” events.

“The heart and soul of this country is at stake,” he said.

Huge early voting

Trump sets the pace with a frenetic schedule, but Biden's strikingly quiet campaign is also revving up.

After Florida, Trump flew to Fort Bragg in North Carolina for a meeting with troops. A planned rally in the toss-up state was postponed until Monday due to the weather, the campaign said.

Mother nature intruded on Biden's Tampa rally too, as a heavy downpour forced him to cut his remarks short.

Trump defeated Clinton in Florida in 2016 but an NBC News/Marist poll released on Thursday had Biden with a 51-47 point lead there.

A stunning 81 million Americans have already cast their ballots early in the hugely consequential election.