10.4m Americans voted so far

US voters have cast more than 10 million votes for the Nov 3 presidential election, significantly outpacing the early vote in 2016 and suggesting a large turnout, according to data compiled by the US Elections Project. 

As of Monday night, nearly 10.4 million Americans have cast a vote in states that report early voting data, according to the election information resource at the University of Florida. The number of ballots cast in five states - Minnesota, South Dakota, Vermont, Virginia and Wisconsin - already exceeds 20% of total 2016 turnout, the Elections Project said.

But the complex rules of mail-in voting also rising worries among both camps.

Three weeks before the US presidential vote, the immense surge in demand for mail-in ballots due to the coronavirus is testing the country's ability to pull off a credible election.

Tens of thousands of ballots in Ohio and New York has been cancelled with the wrong candidate or voter names. Votes thrown in the trash in Pennsylvania over naked votes and scores of lawsuits have been filed over the counts.

President Donald Trump says voting by mail is ripe for fraud and has promised to challenge ballot counts.

"Out of control. A Rigged Election!!!" Trump tweeted on Friday after news that ballots with the wrong candidates were sent to 50,000 residents of Columbus, Ohio, a key state Trump needs to win for reelection.

But election analysts say that, despite those headline cases, so far the process is going well.

Nearly 75 million mail-in ballots have been sent or requested so far this year, more than double the 33 million in 2016, according to the US Elections Project of the University of Florida on Monday.

But errors and delays in getting the ballots printed and mailed, cases of mishandled ballots like in Pennsylvania, and reports of people receiving more than one ballot have stoked attacks on the concept at large.

The Ohio and New York ballot problems were simple printing and envelop insertion errors that could have been avoided with better management.

Republicans especially have tried to limit the impact of mail-in voting, as surveys show Democrats are much more likely that Republicans to mail their ballot rather than vote in person.

In Pennsylvania they battled for a court ruling to throw out ballots if the voter uses the wrong envelope. In South Carolina Republican attorneys won a ruling requiring envelopes for ballots to be signed by witnesses as well as the voter.

The issue of rejected ballots could be crucial.

The 2000 presidential battle between George W Bush and Al Gore was decided by a 537 vote margin in Florida, and hinged on the Supreme Court blocking a broad recount that could have restored thousands of rejected ballots.