In a tight race, the ballots cast when voters don't bring the proper ID to the polls, among other reasons, become crucial to the outcome.
By law, these so-called provisional ballots can't be counted for at least 10 days after the election — Nov. 17, this year — to give Ohio officials time to verify a voter's eligibility.
Recall the drawn out disputes in the presidential contest eight years ago between President George W. Bush and Democrat John Kerry.
The number of provisional ballots cast then was larger than Bush's margin of victory over Kerry — about 119,000. Kerry didn't concede until the next morning. A recount initiated and paid for by third-party candidates took weeks but didn't change the outcome.
With 18 electoral votes, Ohio again is at the center of both candidates' campaign strategies.
And the close race between President Barack Obama and Republican challenger Mitt Romney has some concerned a conclusion could take days.
Some elections officials fear more Ohioans will be required to vote a provisional ballot this year after Secretary of State Jon Husted sent absentee ballot applications to about 6.9 million residents. Those who request the mail-in ballots but decide to vote in person on Election Day will have to vote a provisional ballot.
About 370,000 mail-in ballots — roughly 28 percent of the 1.3 million requested — had not been returned as of last Friday, according to Husted's office. That number is expected to decline as the election nears.
Husted says he can't predict how many provisional ballots will be cast this year. But he has told reporters he expects the state will be able to declare a winner.
"I just believe that we'll have the information necessary on election night to have a degree of confidence as to who won Ohio," Husted said Thursday.
Ohio had a provisional voting system in place before a 2002 federal law required states to have them.
Ohioans cast about 206,000 provisional ballots in the 2008 presidential election — second only to California. Husted's office estimates that 130,000 of those ballots were cast because voters didn't update their address.
Husted, a Republican, has tried to curb the number cast this year by allowing registered voters to update their addresses online.
How certain provisional ballots get counted in Ohio has been part of an ongoing legal dispute.
But on Wednesday, a federal appeals court put on hold a lower court's order that would have required the state to count ballots cast not just in the wrong precinct but in the wrong polling location altogether.
In response to a federal court ruling, Husted has already ordered poll workers to count ballots cast by voters who show up at the correct polling place, but are mistakenly directed to an area where votes for other precincts are being cast.
Other elections in Ohio have hinged on provisional ballot counts.
Democrat Mary Jo Kilroy overcame a 594-vote deficit to beat Republican Steve Stivers by 2,311 votes in a 2008 congressional race after provisional ballots were counted in Franklin County, home to Columbus.
Edward Foley, an elections law expert at Ohio State University, said voters shouldn't assume there's a problem if Ohio's results don't immediately point to a winner in the competitive presidential or U.S. Senate race.
"In a close election, it's to be expected in this new world of provisional voting that some outcomes are just going to have to wait for the evaluation of provisional ballots," Foley said. "That in and of itself is hardly a crisis. It's the system working as designed."