In the wake of the 2012 House elections, it looks like Democrats won a slight majority of the major-party votes (roughly 50.5%) but only about 46% of the seats. A story has gradually developed that pins this gap on redistricting (for example, see here, here, and here), since Republicans controlled the line-drawing process more often than not this time around. Matthew Green pushes back a little on this narrative, arguing that, if anything, House seat share and vote share correspond more closely today than they once did. But he doesn’t necessarily deny that redistricting is to blame for the gap this year.
We have looked at this question several times before (here, here, and here) and concluded that redistricting is a wash. But we based this conclusion on a multi-year model with both incumbency and the partisanship of the constituency (as measured by the presidential vote in each district). What if our model missed something important about the national climate this year? Not crazy, since it was too bearish on Democrats. For that matter, what if all this talk about incumbency is nonsense and it’s all about the district?
Let’s work with those assumptions. We’ll drop our regular model and go bare bones. Two steps: 1) identify the relationship between this year’s actual election returns and the 2008 presidential vote in each district (calculated by Daily Kos), 2) use this relationship plus the 2008 presidential vote in the old districts to estimate what would have happened under the old lines. No incumbency, no assumptions about national climate. For the redistricting story to hold, this exercise must eliminate the discrepancy between Democratic vote share and seat share. Otherwise, something else is going on.
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.