More than 100 million Americans cast their ballots in advance of Tuesday's Election Day, according to the US Elections Project watchdog, a record figure largely attributed to the Covid-19 pandemic.

The ballots, which were mailed in, deposited in drop boxes or cast at polling stations ahead of Tuesday, represent more than 72 per cent of the total number of ballots cast in the 2016 presidential election, according to the tally by the watchdog based at the University of Florida.

In the states of Texas, Hawaii, Washington and Montana the number of early votes exceeded the total number cast there in 2016.

Four years ago, some 57 million voters had cast an early ballot, according to the US Election Assistance Commission.

Of the more than 100.2 million early votes cast this time around, more than 64.5 million are mail-in ballots — an option widely expanded across the country in response to fears of Covid-19 contagion in crowded polling stations.

Several key states — including Michigan, Pennsylvania and Wisconsin — do not begin counting mail-in ballots until Election Day itself, fueling fears a final result could take days.

US President Donald Trump, a Republican, has launched repeat assaults on the credibility of mail-in voting, often claiming without evidence that the process will be rigged against him.

Trump's Democratic opponent, Joe Biden, leads the race in national polls and in many of the battleground states that will decide the high-stakes race.