Just to remind you, if no presidential candidate gets a majority of votes in the electoral college, the election is decided by the House of Representatives, each state delegation casting a single vote for their state. This has only happened twice before, in 1801 (when the election system was different) and in 1825 (where there was a four-way split among the leading candidates in the popular vote). In 1801 it took 36 ballots before Thomas Jefferson was elected; in 1825 John Quincy Adams secured the necessary majority first time round.
Now that there is a strong two-party system, it's hugely unlikely that the House will have to decide in this way. It would require either the 269-269 split of electoral college votes, or enough faithless electors defecting from a narrow winner to take him or her below the 270 threshold (or I suppose the winning candidate dying and no other solution being found). Nevertheless, both in 2016 and 2004 I looked at what might happen if the presidential election were to be decided in the House of Representatives, specifically at the hill the Democrats would need to climb to win in that scenario. In 2020, two years after a decent result for the Democrats in 2018, the hill is not quite as high.
Afte the 2018 election, Republicans had the majority in 26 state delegations to the House, Democrats had the majority in 22, and Michigan and Pennsylvania were evenly split, and that remains pretty much the case now (well, one of Michigan's Republicans elected in 2018 now calls himself a Libertarian, but is not standing again). Assuming that all Democrats hold their seats, and that there is a uniform swing across the USA from Republicans to Democrats from the 2018 results, the states fall as follows:
0.4% swing: Democrats win Kansas 2nd district, splitting the state delegation 2-2 and depriving Republicans of their 26th state and their majority.
(R 25 – 3 – D 22)
(Michelle De La Isla defeats Jacob LaTurner)
1.3% swing: Democrats take Pennsylvania 1st district, flipping the state delegation 10-8 into their column
(R 25 – 2 – D 23)
(Christina Finello defeats incumbent Brian Fitzpatrick)
2.1% swing: Dems take five seats in Texas (23rd on a 0.2% swing, 21st with 1.3%, 31st with a bit less than 1.5%, 24th with a bit more than 1.5% and 10th with 2.1%) splitting the state delegation 18-18 (from 23-13).
(R 24 – 3 – D 23)
(Gina Ortiz Jones defeats Tony Gonzales)
(Wendy Davis defeats incumbent Chip Roy)
(Donna Imam defeats incumbent John Carter)
(Candace Valenzuela defeats Beth Van Duyne)
(Mike Siegel defeats incumbent Michael McCaul)
2.3% swing: Dems take the next seat in Michigan (the 6th district) and also the only seat in Montana, to put them level with the Republicans.
(R 23 – 2 – D 25)
(John Hoadley defeats incumbent Fred Upton)
(Kathleen Williams defeats Matt Rosendale)
2.5% swing: Dems take a sixth seat in Texas (the 22nd district) to get a majority of state delegations.
(R 23 – 1 – D 26)
(Sri Preston Kulkarni defeats Troy Nehls)
And going on a bit further, two more changes require less than a 5% swing:
3.0% swing: Dems take the swing seat in Florida (the 15th district) flipping the state to 14/13 from 13/14
(R 22 – 1 – D 27)
(Alan Cohn defeats Scott Franklin)
3.3% swing: Alyse Galvin ("non-partisan" but supported by Dems) takes the only seat in Alaska.
(R 21 – 1 – D 28)
(Alyse Galvin defeats incumbent Don Young)
Of course, this is assuming that all Democrats hold their seats, and that there is a uniform swing across the USA from Republicans to Democrats; two assumptions that we know will be wrong.
For completeness, the 22 states which currently have Democratic majorities in their congressional delegation are: Arizona, Colorado, Delaware, Vermont, Hawaii, Iowa, Maine, Minnesota, Nevada, New Hampshire, Rhode Island, New Mexico, Oregon, Virginia, Washington, Connecticut, Maryland, Illinois, Massachusetts, New Jersey, New York and California. The 21 states currently with Republican majorities not mentioned above are: North Dakota, South Dakota, Wyoming, Idaho, Mississippi, Utah, Wisconsin, Nebraska, West Virginia, Oklahoma, South Carolina, Arkansas, Kentucky, Louisiana, Missouri, Georgia, Alabama, Indiana, Tennessee, North Carolina and Ohio.
Incidentally, if no vice-presidential candidate gets a majority in the Electoral College, the Senate gets to choose. This has happened once, in 1837, when 23 Democratic electors from Virginia refused to vote for Martin Van Buren's Vice-Presidential candidate, Richard Mentor Johnson, because he had openly cohabited with a black woman. This left Johnson one vote short of a majority in the Electoral College. The lame duck Senate was controlled by his party, and they duly elected him in February 1837 by a large majority – though interestingly with some defection back and forth: one Democratic Senator and three Whigs (including future Vice-President Thomas Hendricks William Hendricks, uncle of future Vice-President Thomas Hendricks) broke ranks and voted for the other party's candidate, while former Vice-President John C. Calhoun abstained.
It's likely that whichever party wins the presidential election this year will also have a majority in the Senate, and if they don't, it would be very weird for the Senate to choose a vice-president of a different party to the President. But weird things have been happening recently.
