That’s an entirely possible scenario—indeed, as of this writing, it’s what the polls currently predict—and it would add up to the slimmest of possible Biden wins, 270 to 268 in the Electoral College. A very close race, but Democrats would celebrate nonetheless.
With such a close margin, though, the incredible and unilateral power to change the election’s outcome would now rest in the hands of Biden’s electors. After all, “we’re not a democracy,” and the 538 electors are the ones who actually elect a president.
Let’s say, in this scenario, that one Biden elector decides to go rogue. Perhaps he’s had secret Trumpist tendencies all along, or thinks the president’s voter-fraud fantasies are real. Perhaps he supported Bernie Sanders in the primaries and has second thoughts about the more moderate Biden. Or perhaps some wealthy donor has quietly offered him $50 million to change his mind.
Whatever the reason, all it would take is the action of a single faithless elector to deny Biden and the Democrats the presidency. If that person wrote down someone else’s name, Biden would drop from 270 to 269 electoral votes, which would mean he’d no longer have a majority.
Under the Constitution, such a scenario throws the election to the House of Representatives, which would then hold a so-called contingent election. Democrats will have a majority of seats in the incoming House—but a contingent election isn’t decided in a traditional House-wide vote. Instead, each state’s delegation votes as a single unit, with the party that holds the majority of a state’s seats getting its way. (For example, Wisconsin will likely have five Republicans and three Democrats in the next House—so its delegation would be expected to vote for a Republican, even if Biden carries the state.) California and Wyoming are again given equal power, despite one having 53 House members and the other having one.
Although dozens of House races have yet to be called, the Republicans will probably control a majority of the state delegations. The GOP currently controls 26 delegations, and Republicans will likely gain seats once all votes are counted. (If all of the current leads in uncalled House races hold, Republicans will control 29 delegations, Democrats will control 20, and one will be tied.)
Those House Republicans would almost certainly vote to give Donald Trump another term—meaning that a popular-vote win and an Electoral College win and a Democratic majority in the House of Representatives still wouldn’t be enough for Biden to take office. All because of one person’s decision.
You may be thinking: Didn’t the Supreme Court solve this? Indeed, this summer, in Chiafalo v. Washington, the Court ruled 9–0 that it is constitutional for a state to require its electors to vote for the candidate who won its popular vote. But that ruling matters only for states that actually have such a requirement—and only 33 states and the District of Columbia do.