Americans are voting early in record numbers with more than 17.8 million casting their ballots already ahead of a presidential election between Democrat Joe Biden and Republican Donald Trump taking place under the shadow of the coronavirus pandemic.

And while the result of the White House race will not be known until Election Day on November 3 -- or perhaps much later -- the early vote indications appear to favor the 77-year-old Biden, who has a double-digit lead over Trump in the national polls.

Iowa, the Midwestern state where the 74-year-old Trump held a campaign rally on Wednesday, is among the states which allow voters to cast their ballots early.

"I did not want to do it by mail," Jeff Maki, a 44-year-old insurance agent, said as he voted early in Newton in Jasper County. "To make sure the vote counted, I wanted to vote early in person."

Election auditor Dennis Parrott said 8,200 early votes were cast in Jasper County in 2016.

"And with three weeks left to go to early vote we have 8,000 now," Parrott said. "So we're going to surpass that record."

Early voting began in Iowa on October 5. As of Thursday more than 325,000 ballots have been cast in the Hawkeye State, according to the US Elections Project of the University of Florida.

The project maintains a running tally of in-person early votes and returned mail ballots. By Thursday afternoon a total of 17.8 million ballots had been cast in the states that report early totals.

Election experts point to the Covid-19 crisis and an expansion of voting by mail for the surge in early voting, which is on a pace to smash the 2016 total of 47 million early votes.

Trump frequently denounces voting by mail as being rife with fraud, but there has been no evidence of widespread irregularities with mail-in votes in past elections.

Michael McDonald, a University of Florida political science professor who runs the US Elections Project, said the early voting numbers bode well for former vice president Biden but there could be heavy turnout for Trump on Election Day.

Several polls have shown that Democrats are more likely than Republicans to vote early because of the Covid-19 pandemic.

"I've warned that the heavily Democratic voting at this point should not be an indicator that Biden has the election [sewn] up," McDonald said in an early vote analysis on his website. "Yes, the numbers are very good for Biden.

"However, it is very likely Republicans will show up in force to vote in person," he added.

 'Cannibalised' 

A Trump campaign spokeswoman told The Washington Post the president was indeed counting on heavy turnout on Election Day.

"For months, Democrats have pinned all their hopes on mail ballots, irresponsibly scared voters away from the polls and cannibalised their Election Day voters in favour of vote-by-mail," Trump campaign spokeswoman Thea McDonald said. "Republicans will show up in person on Election Day and reelect President Trump."

According to the US Elections Project, voters, as of Thursday, have requested 80.69 million mail-in ballots in the states reporting such figures.

Of the states that report requests by political party registration, Democrats have requested 23.66 million ballots and Republicans 13.91 million.

A total of 2.09 million early votes have been cast in Florida, 1.68 million in California and 1.62 million in Texas, according to the US Elections Project.

Other states with high early vote totals include Michigan (1.3 million), Virginia (1.14 million) and New Jersey (1.14 million).

More than 916,000 early votes have been cast in the southern state of Georgia despite long lines on the first day this week which saw some voters waiting for more than 10 hours.

North Carolina began in-person early voting on Thursday, and Democratic vice presidential candidate Kamala Harris had been scheduled to travel to Asheville to address an early vote mobilisation event.

Harris was forced to cancel, however, after her communications director and a flight crew member tested positive for Covid-19

Early in-person voting began on Tuesday in Texas, which has 38 Electoral College votes and is key to Trump's hopes of victory.

Harris County, which includes the major metropolis of Houston, saw a record number of in-person early voters on the first day, when 128,186 cast their ballots -- nearly double the 68,000 who did so four years ago.

The figure was slightly lower on Wednesday -- 114,996 -- but still substantially higher than in 2016.

"I wanted to make sure that my vote is counted before anything can happen between now and Election Day," Afshar said.