One more dorky thing I've been looking at: What happens (as in some cases in PA this year) when the composition of an incumbent's district changes a lot after redistricting? e.g. Say there's only a 40% overlap between her old and new constituents. Is she likely to underperform?
The answer is that this *doesn't* seem to matter all so much. In extreme cases, it might cost her a couple of percentage points. But if the overall partisanship of the new district is about the same, she'll usually perform almost as well, even if she has lots of new constituents.
A *big* qualifier to this is that it's fairly common for incumbents to retire if they get drawn into a new district they really don't like. So there's meaningful survivorship bias. Even so, I'd have guessed there would have been a larger effect.
After looking at this more deeply, I think I may have understated the effect. There's a reasonably large effect from losing your constituents if you're drawn into a competitive district. It doesn't matter as much if you're in a noncompetitive one,.
