About 47.5 million Americans have turned in ballots, roughly eight times the number of early votes cast about same point before the 2016 presidential contest, and above the 47.2 million early votes that were cast before election day in 2016.

The surge comes after many states have expanded mail-in voting and in-person early voting as a safe way to vote during the coronavirus pandemic and amid voter eagerness to weigh in on the political future of Republican President Donald Trump, who is facing a stiff challenge from Democrat Joe Biden.

Biden leads Trump in national opinion polls, although surveys in crucial battleground states indicate a tighter race.

The high level of early voting has led Michael McDonald, the University of Florida professor who administers the US Elections Project, to predict a record turnout of about 150 million, representing 65 per cent of eligible voters, the highest rate since 1908.

Trump has railed against mail-in voting, making unfounded accusations that it leads to fraud. Experts have said such fraud is rare.

Those attacks by the president have shown signs of depressing Republican interest in voting by mail. Democrats have roughly doubled the number of returned mail-in ballots by Republicans in states that report voter registration data by party, according to the Elections Project.

-AAP