If the U.S. is still waiting to learn who the president will be days or even weeks after Tuesday’s election, provisional ballots are likely to be at the centre of any disputes.

They are the ballots cast by voters whose eligibility is questioned for some reason. Those ballots are set aside and held for a period of days after the election while workers determine whether they should be counted.

Experts say the number of provisional ballots this year may set a record, exceeding the 2.7 million cast in 2012 and almost 2.5 million cast in 2016, in part because some voters who requested ballots by mail are showing up at polls to vote in person. That could lead to late results in key battleground states such as Pennsylvania -- where President Donald Trump won by only 44,292 votes in 2016 -- if the race is close and the source of challenges and litigation.

“It’s just another one of those things that plays into the unusual nature of this election,” G. Terry Madonna, director of the Centre for Politics and Public Affairs at Franklin and Marshall College in Pennsylvania, said of provisional ballots.

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Provisional ballots have come into play in the past. Democrat John Kerry held off conceding the 2004 presidential race to Republican George W. Bush until the morning after the election because he waited until it was clear the 157,714 provisional ballots cast in Ohio couldn’t change the outcome.

In 2016, Trump carried Arizona by 91,234 votes, and there were 102,510 provisional ballots, according to the U.S. Election Assistance Commission. Pennsylvania had 26,451 provisionals, and that was before the commonwealth expanded voting by mail last year.

Greater use of mailed ballots, coupled with disruptions in mail service, may contribute to the bumper crop of provisional ballots. After initially urging voters to request mail-in ballots in response to the pandemic, Democratic leaders have been telling people to vote in person or deliver the ballot to an elections office or an approved drop box to avoid the risk of it being disqualified on technical grounds or having its delivery delayed by the U.S. Postal Service.

In most states, voters who received a mail-in ballot but show up at a poll instead will cast a provisional ballot unless a jurisdiction uses an electronic poll book or allows voters to bring their mail-in ballot to be canceled so they can cast a regular ballot, said Tammy Patrick, a senior adviser on elections at the Democracy Fund.

Election officials in states with early voting are reporting significant numbers of voters who requested a mail-in ballot showing up to cast a ballot in person. For example, in Franklin County, Ohio’s most populous, about 11 per cent of the voters who have cast an in-person vote so far had also requested an absentee ballot, according to the county Board of Elections.

The extra time it takes to cast a provisional ballot will make voting waits longer if there are long lines, and election offices should be preparing for that, said David Becker, executive director and founder of the Centre for Election Innovation & Research.

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Some Democrats are concerned that Trump, who has sought without evidence to discredit mail-in voting and ballots counted after the election, will declare victory based on an early lead on election night from in-person votes before outstanding mail-in and provisional ballots that favor Democrats are counted.

That could result in a “big blue shift” if Trump has an election-night lead that changes when votes favoring Democrats and Biden are counted. Data show Democrats have requested more mail-in ballots that Republicans. Of the 2.1 million ballots returned so far in Pennsylvania, 68 per cent are from Democrats.

“Any sizable number of uncounted ballots should be a concern for everyone,” said Michael Nutter, a former Philadelphia mayor and a Democrat.

The Biden campaign is confident the voting process will go smoothly and has put in place an “unprecedented effort to both protect voting rights and ensure voters have the information they need to cast regular ballots,” spokesman Michael Gwin said.

The Trump campaign didn’t address provisional ballots specifically but said the enthusiasm for Trump, the campaign’s get-out-the vote efforts and gains in Republican registration in states such as Pennsylvania will carry the president to victory.