These numbers are more important than ever for understanding U.S. elections. Due to increased political polarization and the decline of split-ticket voting, there's now a very strong correlation between how a given district votes in presidential races and how it votes for the House. We can see this most vividly in the long-term decline of so-called "crossover" districts, which support one party's candidate for president but the other's for House.
Following redistricting, just 14 Democratic-held seats would have voted for Donald Trump while only 15 Republican-held seats would have backed Joe Biden, as shown in the chart below:
That total of 29 is the third-lowest in a century. The lowest figure in that timeframe came just prior to redistricting, when there were 16 crossover seats. The increase is to be expected, since both parties sought to redraw maps in ways favorable to them (albeit with Republicans doing so in more states and more aggressively), but the totals are likely to shrink again following the midterms.
Armed with these district-level presidential results, then, we can identify which races are likely to be competitive, and we can predict the outcome of most House elections with a strong degree of confidence. In an era when the reliability of polling has seemed uncertain, this hard data offers an important alternative. Of course, it's not the be-all, end-all, which is precisely why Daily Kos Elections tracks campaign news so closely and shares it each day in our Morning Digest newsletter, but it's a critical component of all good House election analysis.
There are many other things this new data tells us, too. At the highest level, we know that Biden would have carried 226 districts while Trump would have prevailed in 209, a slight increase from Biden's 224-211 edge prior to redistricting. But that topline obscures the way in which Republican gerrymandering has increased the GOP's advantage on what was already a playing field tilted in its favor.
We know this because the single most important fact this data reveals is the identity and characteristics of what's known as the "median district." A helpful way to assess how much the national congressional map favors one party or the other is to sort each seat by Biden's margin over Trump (or vice versa) and see how the seat in the middle—the median seat—voted.
That district is now Michigan's 8th, a Democratic-held seat based around the city of Flint that would have voted for Biden by a 50-48 margin. On the surface, that might seem like welcome news for Democrats: If they were to capture every seat bluer than the 8th—a useful framework for illustrative purposes—they'd win a majority in the House.
But this district is in fact to the right of the nation as a whole. Biden won the national popular vote by a margin of just under 4.5 points, so his performance in Michigan's 8th (where his margin of victory was a shade over 2 points) is 2.4 points worse. With the House so closely divided, those 2.4 points could make all the difference. What's more, this new median is further to the right than the pre-redistricting median, Illinois' old 14th District, which was 2 points worse than Biden's national vote.
Of course, even in a wave election, Democrats almost certainly would not win every district bluer than Michigan's 8th—there will always be some crossover seats (the lowest since 1900 was five, in 1904). But looking at the numbers this way illuminates the built-in advantage that Republicans have before even a single vote is cast, and how that advantage grew this year thanks to gerrymandering.
There's much more, especially at the level of individual districts. For instance, the bluest seat that Republicans are defending is Texas' 34th in the eastern Rio Grande Valley, which would have voted for Biden by a 57-42 margin. There's an asterisk here, though, since two incumbents are running against one another: Republican Rep. Mayra Flores, who flipped the old 34th in a special election earlier this year, and Democratic Rep. Vicente Gonzalez, who opted to leave behind his old 15th District next door and run in the bluer 34th.
If you're looking for a more straightforward answer, you'll find it in California's 22nd in the southern Central Valley, which would have backed Biden 55-42. Rep. David Valadao is consequently one of the most vulnerable Republicans this year and faces a stiff challenge from Democratic Assemblyman Rudy Salas. Valadao is used to the headwinds, though, as his old district supported Biden by a similar 54-44 spread.
His Democratic counterpart would be Rep. Tom O'Halleran, who saw his sprawling 2nd District in northeastern Arizona become considerably redder, going from 50-48 for Biden to 53-45 for Trump. That dramatic shift came about because the independent tiebreaker on Arizona's redistricting commission voted for a GOP plan that, well, favored the GOP.
Some other notable superlatives that emerge from the data:
Closest seat in the nation: Arizona's 6th, 49.3-49.2 Biden—a margin of 0.1%, or about 300 votes
Reddest seat in the nation: Alabama's 4th, 80-19 Trump
Bluest seat in the nation: Maryland's 4th, 90-9 Biden (Biden won a slightly higher vote share but a slightly smaller margin in Pennsylvania's 3rd, which was also 90-9)
Reddest Democratic-held open seat: Georgia's 6th, 57-42 Trump
Bluest Republican-held open seat: Illinois' 13th, 54-43 Biden
District that got the most redder: Tennessee's 5th, 55-43 Trump (vs. 60-37 Biden)—a shift of 35 points
District that got the most bluer: California's 8th, 76-22 Biden (vs. 55-43 Biden for its predecessor, numbered the 3rd)—a shift of 42 points
If you dive in yourself, you'll find so much more. Once again, you can find all of our data right here, and we strongly encourage you to bookmark the link. And if you'd like to play with the numbers yourself, we've also made a spreadsheet version available. Please let us know what you discover!