More than 10 million Americans have already cast their ballots in the US presidential contest, a tracking group said late Monday, a record pace three weeks before election day November 3.

"Voters have cast a total of 10,296,180 ballots in the reporting states," the US Elections Project of the University of Florida reported on its website.

The project said the early-vote count is multiple times higher than it was at the same point in 2016, fuelled by dramatic surges of mail-in voting due to health concerns about casting ballots in person during the coronavirus. 

 - Mail-in voting -

Tens of thousands of ballots in Ohio and New York with the wrong candidate or voter names. Votes thrown in the trash in Pennsylvania and scores of lawsuits over the counts.

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.

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. 

"There's going to be execution errors along the way," said Kevin Kosar, a political analyst at the American Enterprise Institute.

"The good news is, these mistakes are happening, but they are being caught early," he said.

- Primary horror stories -

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.

Horror stories from the primaries earlier this year, particularly in New York and Wisconsin, support the concerns. 

In both states tens of thousands of people either did not get their mail ballots or received them too late to vote. The postal service did not put postmarks on thousands of ballots to show they had been mailed on time. 

Amber McReynolds, chief executive of the National Vote At Home Institute, said a few cases have blown up in the press and social media, but "are not widespread."

"It's 2020 and everybody is highly sensitive," she said.

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

"Is everything going to be perfect all the time? No. There never has been a perfect election," McReynolds said.

"If we got two issues in two counties out of 8,000 jurisdictions, then the bigger story is it's working effectively in the majority of places."

McReynolds, former head of elections in Denver, Colorado who helped set up the state's universal by-mail voting system, said everyone is focused this year on mailed ballots.

But in-person voting has its own problems, she argued, especially machine breakdowns and long lines, like those seen in Georgia on Monday, a particular issue given the Covid-19 pandemic.

"I believe that there are actually more problems in in-person voting," she said.

- Lawsuits -

Still, there will be problems on the November 3 election day. 

McReynolds said three key battleground states that could decide the winner -- Wisconsin, Pennsylvania and Michigan -- haven't given themselves enough time to open, verify, and then count millions of mailed ballots.

Kosar says what worries him is the possibly hundreds of lawsuits being pursued over how mailed ballots will be counted, as well as other process issues.

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 Republican George W. Bush and Democrat 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.