Most projections of possible Electoral College outcomes in the 2020 presidential election use 270 as the minimum number of electoral votes needed to win the presidency. Two-hundred-seventy is the smallest possible majority out of the total of 538 electoral votes.  However, for President Trump, the minimum number of electoral votes needed could well be 269 rather than 270.
If Trump were to receive 269 electoral votes, with no electoral votes being cast for a third-party candidate, the Democratic presidential candidate would also have 269 electoral votes.  One possible outcome that would produce a 269-to-269 electoral vote tie would be for the Democratic candidate to win all the states that Hillary Clinton won in 2016, plus Michigan, Wisconsin and Arizona. 
All three of these states are ones in which Democrats won competitive statewide races — for governor, U.S. Senator or both — in the 2018 midterms, so it is not unreasonable to expect a Democrat could win the presidential popular vote in those states next year.  There are several other combinations of electoral votes that could also produce a 269-to-269 tie.
So, what happens if the electoral vote is evenly divided?  The answer is provided by the Twelfth Amendment to the Constitution.  If, when the electoral votes are formally counted in a joint session of Congress, no candidate has a majority, the House of Representatives would elect the president, with each state delegation having one vote, and a majority of states needed to elect.  In other words, Vermont’s one House member would have an equal vote with California’s 52 House members, and the votes of 26 state delegations would be necessary to elect a president.
If there were a tie in the Electoral College, the House to be elected in November 2020 would elect the President in early January 2021, as one of its first orders of business after the members had taken their oaths of office.  In today’s polarized political environment, the best assumption in this scenario would be that House members would vote for the presidential candidate of their political party, regardless of who won the popular vote in their states, or nationwide.
What can we say today about the likely composition of the 50 states’ House delegations in January 2021?  Currently, Republicans control 26 House delegations, Democrats control 22 delegations, and two delegations — Michigan and Pennsylvania — are evenly divided.  As long as the GOP can continue to control at least 26 House delegations, Trump would likely be re-elected if the electoral vote ended up in a 269-to-269 tie, even if the Democratic candidate won the popular vote by a margin of several million.
Most House members running for another term are re-elected.   Most open seats remain in the hands of the party that previously held them.  However, there are a few closely divided House delegations that will be the focus of intense campaign activity in 2020. 
If Democrats could win one more seat in each of the Michigan and Pennsylvania delegations, and hold on to all the delegations they currently hold, they would have majorities in 24 state delegations.  To get from 24 to 26 delegations, Democrats would need to flip one seat in Florida (now divided 14-to-13 for the GOP), and two seats in Wisconsin (now 5-to-3 Republican).  This will not be easy, as both Florida and Wisconsin are well-known for pro-GOP gerrymandering and voter suppression.
The Republicans’ top targets, in terms of flipping state delegation control, will be Arizona (now 5-to-4 Democratic) and Colorado (now 4-to-3 Democratic).
Although the probability of an electoral college tie in 2020 is not very high, it is not trivially small.  Both parties’ House campaign organizations will thus be working very hard to maintain the current partisan balance of state delegations, in the case of the Republicans, and to flip at least four delegations, in the case of the Democrats.
Eric L. Davis is professor emeritus of political science at Middlebury College.