With 538 electoral votes, the Electoral College does contain the possibility of a 269-to-269 tie. Could we run into this situation in the 2024 presidential race? It's possible, though pretty unlikely: According to 538's presidential forecast, there's only about a 1 in 450 chance neither Harris or Trump attain 270 electoral votes. Still, that's greater than zero, so let's look at how this could play out.
If Congress finds the absence of a majority when it gathers on Jan. 6, 2025, to certify the Electoral College results, the Constitution calls for the selection of our national executives via a "contingent election." In that scenario, the 12th Amendment commands the U.S. House of Representatives to "immediately" choose the president from no more than the three candidates who won the most electoral votes — so in a 269-269 tie, that would mean just the two major-party nominees. However, the choice is not based on a simple vote by the House's 435 members as a whole, but instead is decided through a unique system whereby each state casts one vote according to the preferences of each member of that state's House delegation — with states that have tied delegations potentially unable to cast a vote unless a member breaks with their party. To win, a candidate must win a majority of state delegations (26).
Under these rules, Trump would likely hold an edge in a contingent election for president. 538's House forecast shows a tight race for overall control of the House (which, of course, is based on which party controls more individual seats), but Republicans are far more likely than the Democrats to have an edge in the House delegations from at least 26 states in the next Congress. Currently, Republicans hold a majority of the House seats in 26 states compared with the Democrats' 22, while two others (Minnesota and North Carolina) are tied. Yet if we take 538's forecast and rate each state based on the race rating of its most competitive seat that would change party control, the GOP looks favored to control as many as 29 state delegations to Democrats' 20 come January, while Minnesota remains likely to have a tied delegation.
Meanwhile, the Senate would decide who fills the vice presidency if no vice presidential candidate earns a majority in the Electoral College. Unlike the House, a Senate contingent election vote would be based on a vote of the chamber as a whole, in which a candidate needs 51 of 100 votes to win. Additionally, the Senate can only consider the top two recipients of electoral votes, so even if a faithless elector casts a vote for another vice presidential candidate, the Senate would find itself picking between Vance and Walz.
The Senate math favors Republicans to win a contingent election for vice president, too, due to their 9-in-10 chance of capturing control of the upper chamber. Currently, Democrats hold a 51-to-49 majority, including the four independents who caucus with them. But Republicans are essentially guaranteed to pick up a Democratic-held seat in dark red West Virginia to get to 50-50. Next, Democratic Sen. Jon Tester is an underdog to win reelection in Republican-leaning Montana, while Democratic Sen. Sherrod Brown is in a toss-up race to keep hold of his Ohio seat. Democrats are also fighting to defend five seats in key presidential swing states Arizona, Michigan, Nevada, Pennsylvania and Wisconsin, which could also bear some fruit for the GOP. On top of this, Democrats have only a small chance of capturing GOP-held seats in states like Texas and Florida to make up ground elsewhere.