US President Donald Trump and Democratic challenger Joe Biden have one last chance to make their case to voters in critical battleground states ahead of the country going to the polls.

Monday is the final full day of a campaign that has laid bare their dramatically different visions for tackling the nation's pressing problems and for the office of the presidency itself.

More than 93 million people have already voted and each campaign insists it has a pathway to victory, though Mr Biden's options for picking up the 270 Electoral College votes needed to win are more plentiful.

However, Mr Trump is banking on a surge of enthusiasm from his most loyal supporters.

Heading into the closing 24 hours, Mr Trump and Mr Biden each painted the other as unfit for office and described the next four years in near apocalyptic terms if the other were to win.

The incumbent president told a rally in Iowa: "The Biden plan will turn America into a prison state locking you down while letting the far-left rioters roam free to loot and burn."

Mr Biden said America was on the verge of putting "an end to a presidency that's fanned the flames of hate".

Speaking in Philadelphia, the biggest city in a state that could decide the presidency, he said: "When America is heard, I believe the message is going to be clear: It's time for Donald Trump to pack his bags and go home.

"We're done with the chaos, the tweets, the anger, the hate."

As the candidates close out the campaign, the pandemic, which has killed more than 230,000 Americans and cost nearly 20 million to lose jobs, reached a new peak in infection rates, threatening yet another blow to lives and livelihoods of voters.

The election caps an extraordinary year that began with Mr Trump's impeachment, the near collapse of Mr Biden's candidacy during the crowded Democratic primary and then was fully reshaped by the coronavirus outbreak.

A record number of votes have already been cast, through early voting or postal ballots, which could lead to delays in their tabulation.

Mr Trump has spent months claiming without evidence that the votes would be ripe for fraud while refusing to guarantee that he would honour the election result.

In the starkest terms yet, Mr Trump threatened litigation to stop the tabulation of ballots arriving after Election Day.

As soon as polls closed in battlegrounds such as Pennsylvania, Mr Trump said, "we're going in with our lawyers".

It was unclear precisely what Mr Trump meant.

There is already an appeal pending at the Supreme Court over the counting of absentee ballots in Pennsylvania that are received in the mail in the three days after the election.

The state's top court ordered the extension and the Supreme Court refused to block it, though conservative justices expressed interest in taking up the propriety of the three added days after the election.

Those ballots are being kept separate in case the litigation goes forward. The issue could assume enormous importance if the late-arriving ballots could tip the outcome.

Under the shadow of possible legal battles, Pennsylvania loomed as most important battleground on the map.

Republican strategist Alice Stewart said the pandemic, the economy and race relations in America have all coincided in unprecedented ways, but that Election Day's outcome will not bring an immediate fix no matter what happens.

"If 2020 is the most consequential election of our lifetime, heaven help us for 2024," Ms Stewart said. "I'm calling Noah and will start building the ark."