Merely voting and counting the votes in this year’s election will be an extraordinary challenge. The country faces the worst public-health crisis in a century, a potentially severe shortage of poll workers, mail-in voting on an unprecedented scale, mounting functional problems at the U.S. Postal Service (with many alleging a plot to undermine voting by mail), and a president already dismissing mail-in voting as fraud-ridden. And all of this is taking place in one of the worst climates of partisan polarization and distrust in American history. To say that this threatens American democracy is no overstatement.

An additional factor could whip these variables into an outright constitutional crisis—one in which both Donald Trump and Joe Biden would claim to have been elected president, undertaking rival preparations for inauguration on January 20: The country’s laws on presidential elections provide a shockingly inadequate guide for resolving an Electoral College dispute. Twice in the past century and a half (in 1876 and 2000), the country narrowly averted a catastrophic deadlock over the presidential-election outcome. We may not be so fortunate in 2020.

Although multiple possible scenarios for a post-election crisis exist, the most plausible one will revolve around how long counting the votes in closely contested states might take. Remember those three states that, by a total margin of 87,000 votes, gave Trump an Electoral College victory in 2016? Michigan, Pennsylvania, and Wisconsin will again be hotly contested in 2020, in addition to a few other states such as Florida and North Carolina. The election result could ride on who wins one or two of these states, as it did in 2000.

Imagine that the presidential race in these states is still not clearly decided when, on December 8, the federal “safe harbor” period closes and states must certify who has garnered their electoral votes. Some mail-in ballots could remain to be counted, or disputes might persist about the validity of some ballots. Perhaps after trailing on Election Night, Biden has come from behind to take a lead as the mail-in ballots are counted. Following Trump’s warnings, Republicans might claim fraud. Or perhaps controversy arises over how closely the signatures on mail-in ballots match those in the voter-registration database. Then what?

Some have speculated that confusion will reign to the point that (under the Twelfth Amendment, ratified in 1804) the president will be elected by the House (with each state delegation casting one vote) and the vice president by the Senate. The much more likely scenario, however, is that partisan politics will drive decisions in each state—and quite possibly dueling decisions in some states. In Florida, Republicans hold both the legislature and the governorship; at least they could probably agree on whom to declare the winner (although the courts might step in with a divergent answer). But in Michigan, North Carolina, Pennsylvania, and Wisconsin, the legislatures are Republican and the governors are Democrats. In any of these states, a close election with an unresolved outcome could produce a scenario in which the state legislature certifies Trump’s electors and the governor certifies Biden’s electors—and both candidates claim to have been elected president.

If this happens, the situation will be so much worse than Bush v. Gore, which ended the contest over the 2000 election in December, long before it risked engulfing the nation in two conflicting claims to the presidency on January 20.

It could even be worse than the Hayes-Tilden election of 1876, which was settled two days before the inauguration. That was a close call, much too close for comfort, but a last-minute compromise (which ended Reconstruction) averted outgoing President Ulysses S. Grant’s biggest nightmare: the threat of two simultaneous inauguration ceremonies, and thus two commanders in chief demanding allegiance from the Army’s generals.