Speaker Nancy Pelosi has begun mobilizing Democrats for the possibility that neither Joe Biden nor President Donald Trump will win an outright Electoral College victory, a once-in-a-century phenomenon that would send the fate of the presidency to the House of Representatives to decide.

Under that scenario, which hasn’t happened since 1876, every state’s delegation gets a single vote. Who receives that vote is determined by an internal tally of each lawmaker in the delegation. This means the presidency may not be decided by the party that controls the House itself but by the one that controls more state delegations in the chamber. And right now, Republicans control 26 delegations to Democrats’ 22, with Pennsylvania tied and Michigan a 7-6 plurality for Democrats, with a 14th seat held by independent Justin Amash.

A battle inside the House could be brutal, even more politically bare-knuckled than Trump and Senate Republicans pushing through a Supreme Court nominee days before the election. In some states, a single seat could decide the partisan makeup of a delegation. There could be extended legal challenges over declaring victors in House races, as national party leaders and their legal teams dive headlong into the results for individual races at the county or even precinct level.