Joe Biden has the lead in Arizona, Michigan, and Nevada. If wins all three, he’ll have exactly 270 electoral votes—the magic number to take the White House.

But that alone might not guarantee a win. Members of the Electoral College—with the exception of Maine and Nebraska, which allocate by congressional districts—are pledged to vote for the highest vote getter in their state. However, in many states, nothing stops them from breaking that pledge and voting for another candidate. These electors who break that pledge are called faithless electors.

In 2016, a total of 10 electors cast faithless ballots, with seven—the highest since 1912—actually counting. Trump lost two electoral votes, and Hillary Clinton lost five. That wasn’t enough to change the outcome, but it could this time.

Let’s say Biden wins Arizona, Michigan, and Nevada, while Trump takes Georgia, North Carolina, and Pennsylvania. In that scenario Biden would have 270 electoral votes to Trump’s 268. It would take only one faithless Biden elector to cause a tie, 269 to 269.

If neither candidate reaches 270 electoral votes, the vote would move to the House of Representatives. But instead of voting as individual representatives, they’d have to vote as a state—with each state getting one vote. While Democrats have the most House seats, Republicans currently control the most state delegations. The President would be favored in such a scenario, as long as representatives voted along party lines.

Trump has already—without evidence—questioned the fairness of the voting process. But it’s uncertain if he would make an unprecedented request for Biden electors to cast “faithless” votes.

Some state laws are on the books to prevent faithless electors: Electors in 14 states, including Iowa and Michigan, would see their vote canceled if they don’t cast it for the statewide winner, according to fairvote.org. Those state laws were upheld by the U.S. Supreme Court in July when it ruled on the Chiafalo v. Washington case. The Supreme Court also deemed state penalties on unfaithful electors as legal. Five states penalize unfaithful electors with fines.

The Constitution gives state legislatures the power to determine how electors for their state are chosen. In total, 538 electors make up the Electoral College. They’ve never changed the final outcome for President. However, in 1836, they nearly changed the vice president outcome—which electors are also required to vote for. Enough electors flipped from Democratic nominee Richard Johnson that it forced the vice presidential race to the Senate—where he was confirmed. If none of the vice presidential candidates top 270 electoral votes, the Senate gets to pick.

If Biden wants to win without worrying about faithless electors, he’d likely need to claim at least one of the following: North Carolina, Georgia, or Pennsylvania—in addition to Arizona, Michigan, and Nevada.