The overriding factor: a Republican president who political mapmakers could not have foreseen at the beginning of the decade. Trump altered the two parties’ coalitions in ways that specifically undermined conventional wisdom about the House map, bringing more rural voters into the GOP tent while driving away college-educated voters.

The trade worked in some states. But it was a Republican disaster in the House, where well-off suburbs, once the backbone of many GOP districts, rebelled against Trump in 2016 and then threw out House members in 2018.

“It’s worth pointing out that the map is still quite gerrymandered,” said David Shor, head of political data science at Civis Analytics, a Democratic firm. “But I think an underappreciated aspect of that is you had districts that elected incumbents that were good fits for the [Republican] coalition that existed — but no longer worked as well when the 2016 realignment happened.”