As votes are tallied, analysts use statistical models to help project the winners. The models compare current votes in a race with results from previous elections.
Is Obama doing as well in central Ohio as he did on 2008? Is Romney doing better than John McCain did in the Cleveland area?
The closest races will come down to a county-by-county analysis of the remaining votes. If Romney is ahead in Ohio, are there enough votes left to be counted in Democratic-leaning counties for Obama to catch him? And what about the votes that don't get counted until after Election Day?
In 2008, nearly 12 percent of the votes in Ohio were counted after Election Day. Among them were 207,000 provisional ballots cast by people who weren't allowed to cast regular ballots because their names didn't show up on voter rolls or they had moved without updating their voter registrations. Ultimately, 81 percent of the provisional ballots in Ohio were counted in 2008.
Ohio's provisional ballots didn't affect the outcome of the presidential election in 2008 because Obama beat McCain by a relatively comfortable margin. But what if this year's presidential election comes down to Ohio — again — and there are fewer than 200,000 votes separating Obama and Romney?
It might take a few weeks to figure out who won the White House, after all.
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.