If the presidential race comes down to Ohio, provisional ballots could end up a focal point of the election — and drag out the results beyond Tuesday night. Washington Post reporters David Fahrenthold and Bill Turque explain what exactly these ballots are, and why they could have such an impact:

In Ohio, a new dispute has broken out over the validity of provisional ballots. Usually, such special ballots — cast by voters but set aside for examination later — are required when something about the voter’s eligibility is in doubt. For example, the voter might lack proper identification or be in the wrong precinct, or the person might have requested an absentee ballot but then showed up to vote in person at a polling place.

When examined in more detail later, provisional ballots are either discarded or, if the voter’s eligibility is established, counted.

The fight over those ballots has now increased the possibility that — if Tuesday’s election comes down to the Buckeye State, it won’t end on Tuesday night at all.

The current fight over provisional ballots resulted in part because of a move by Ohio’s secretary of state, as Rick Hansen, an expert on election law, explained to Wonkblog’s Sarah Kliff:

A new wild card this year is the [Ohio] secretary of state sent out absentee ballot applications to everyone in the state. Anyone who asked for an absentee ballot, but hasn’t returned one, if they go to the polls, they will have to cast a provisional ballot. This could add thousands of people, casting provisional ballots which won’t be counted until November 17.

Post reporter Rosalind Helderman took a look at how many of these provisional ballots might be in play:

There were 150,000 such provisional ballots cast in 2008. There could be at least that many this year — including those cast by voters who requested an absentee ballot but didn’t return it. State rules say such people cannot use standard ballots on Election Day and must vote provisionally.

Matt McClellan, a spokesman for Secretary of State Jon Husted (R), said election boards have been instructed to report the number of provisional ballots cast in their counties before concluding their work Tuesday or early Wednesday.

That means that by the end of the Election Day counting, it will be known which candidate is ahead and how many provisional ballots remain to be counted.

Provisional ballots tend to be used more often by low-income and transient voters, and both sides assume they will break strongly for Obama.

That means Romney could hold a small lead in Ohio at the end of the counting on Tuesday and still lose the race there — possibly even decisively — if several hundred thousand provisional ballots remain to be counted.

Under that scenario, the public should prepare for a long wait. Ohio rules say provisional ballots cannot be counted until 10 days after an election.

To see election results for Ohio as they come in — as well as for every other state — check out the Washington Post’s election map, updating live throughout night.