Donald Trump won 30 states in 2016—enough for 306 electoral votes—while Hillary Clinton’s haul of 20 state victories would equate to 232 electoral votes.*

But that didn’t happen. When it came time for the Electoral College to vote, Trump lost two electoral votes and Clinton lost five. In Washington alone, four electors passed on Clinton: Three voted for former U.S. secretary of state Colin Powell, the other for Faith Spotted Eagle, a Native American activist.

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 most states, nothing stops them from breaking that pledge and voting for another candidate. And there is a name for them: faithless electors. A total of 10 electors cast faithless ballots in 2016, with seven—the highest since 1912—actually counting.

Those seven faithless electors weren’t numerous enough to swing the election—it takes 270 electoral votes to win—but faithless electors could swing the election in 2020 if it’s close between Trump and Democratic nominee Joe Biden.

Biden is currently leading polling averages in eight states Trump won four years ago: Arizona (+3.9 points), Florida (+1.4 points), Georgia (+1.2 points), Iowa (+1.2 points), Michigan (+7.2 points), North Carolina (+2.7 points), Pennsylvania (+4.4 points), and Wisconsin (+6.1 points).

If Biden pulls off victories in Michigan, Pennsylvania, and Wisconsin, he’d have enough for 278 electoral votes. But let’s say nine electors in states Biden wins don’t vote for the Democratic nominee or Trump. That would mean neither candidate hit 270 electoral votes. In a scenario where none of the candidates reach 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. As of today, Trump would be favored in such a scenario, as along as representatives voted along party lines.**

There are some laws 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.

State legislatures have the power to determine how electors for their state are chosen. In all, 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.

As of Monday, FiveThirtyEight forecast the odds of Biden winning at 88%, while The Economist forecast the Democratic nominee having a 91% chance of winning. Biden holds a +8.9 point national polling average, according to RealClearPolitics. Clinton held a +5.5 national polling average on this same day four years ago.

*In 2016, Trump won 30 states and Maine’s 2nd congressional district. Clinton won 20 states and the District of Columbia.

**This scenario includes Trump winning Maine’s 2nd congressional district and Nebraska’s 2nd congressional district, which FiveThirtyEight rates as being to the political right of Michigan, Pennsylvania, and Wisconsin.