I’d imagine gerrymandering was a bigger factor in PA than in the nation overall, but Democrats should be aware that redistricting is not the only reason Republicans kept the House. Eric McGhee suggests the other factors were incumbency and an “inefficient” concentration of Democrats in urban areas. The culprit is single-member districts. It’s a crazy long-term goal, but it’s worth keeping in mind that electing members of Congress through party-list proportional representation would fix both of these problems by diminishing the importance of geography and increasing the importance of parties relative to candidate effects.
Results are in the graph below. Democrats do gain more seats under this simulation—seven more total—but fall far short of matching their predicted vote share. The point should be clear: even under the most generous assumptions, redistricting explains less than half the gap between vote share and seat share this election cycle.
And it’s worth noting just how generous these assumptions are. This bare-bones model misses more individual outcomes than any handicapper or other forecasting model. It ignores one of the most important ways that a gerrymandering party tries to stick it to the other side (i.e., by moving incumbents to more difficult territory), as well as one of the most important ways that a gerrymander doesn’t work (i.e., incumbents beat expectations based on district partisanship alone). It’s not really a bad model. But it’s not really a good one, either.
If redistricting doesn’t explain the discrepancy, what does? We have argued that incumbency is a likely culprit, but as Dan Hopkins recently pointed out, Democrats also do worse because they are more concentrated in urban areas. They “waste” votes on huge margins there, when the party could put many of those votes to better use in marginal seats. (See this paper by Jowei Chen and Jonathan Rodden for more evidence on this point.)