John Fund explains at National Review Online why it’s possible that the election won’t be over tonight.

“Margin of litigation” is a phrase we might hear a lot after Tuesday if the presidential election is close in one or more states. It refers to the number of votes by which a candidate must win in order to prevent litigation — and delays — before a result is final. Both parties will have armies of lawyers deployed on Election Night looking for irregularities and opportunities to go to court. On Sunday, Trump running mate Mike Pence told Fox News Sunday that “a clear outcome, obviously both sides will accept.” But then he went on to say, “I think both campaigns have also been very clear that, you know, in the event of disputed results, they reserve all legal rights and remedies.”

We almost went into a Bush v. Gore–like election overtime in 2004 because of provisional ballots. Voters in every state must be given a provisional, or conditional, ballot if for any reason they are unable to cast a regular ballot (if, for instance their name is not on the voter rolls or they lack voter ID, or if an election official brings a challenge). The provisional ballot is then cross-checked with public records to see if it’s valid.

In 2004, George W. Bush won nationwide by 3 million popular votes, but his Electoral College majority hinged on his winning Ohio. On Election Night, he had a 136,000-vote lead over John Kerry in that state, with 159,000 provisional ballots left to be examined. Two days after the election, the New York Times reported that election-law specialists had told Kerry that “the number of votes that might have been harvested using all the legal tools available to him could not bridge that gap.” So Kerry conceded on the Wednesday morning after the election, even though his running-mate, John Edwards, and others were urging him to go to court over other alleged irregularities. (Bush’s final certified margin in Ohio was just over 118,000 votes.)

This year, the election could be close enough in one or more states to bring one or both sides into court. “The risk of that happening is higher than it used to be — and higher than most of us realize,” Edward B. Foley, the director of an election law center at Ohio State, and Charles Stewart III wrote in the Washington Post today. They note that votes counted after Election Day can easily determine the outcome of a close election. In 2012, President Obama’s final victory margin exceeded his Election Night margin by more than 20,000 votes in several battleground states.