Apportionment & Redistricting

How have apportionment and redistricting changed the congressional landscape? The new maps suggest a very small net gain for Democrats, but with re-apportionment still shy of even net proportional. Moreover, an approximate national balance obscures offsetting maps in a handful of states that are more unfair.

Details

The table below compares the maps used in the 2020 elections with the new maps that will be used in November. Both sets of maps are compared using the same election composite which combines several statewide elections to remove the effects of individual candidates and campaigns. Hence, these comparisons isolate the impacts of the new apportionments and district boundaries.

• The “DEM %” column shows the composite’s statewide two-party Democratic vote share for each state. The corresponding Republican vote share is simply the complement and could be used instead.
• The “Apportionment” section shows the previous and new numbers of congressional districts for each state and which gained and lost seats.
• The “2020 Maps” section shows the actual number of Democrats elected, the cumulative fractional seat probabilities (Sf) for the statewide vote shares, and the number of whole seats that would be closest to proportional (PR). By the end of the decade, the congressional maps were in aggregate, roughly balanced.
• The “2022 Maps” section shows the corresponding information for the new maps. Due to re-apportionment though, PR for the same statewide vote shares implies that Democrats should pick up a few seats (3 = one-seat gains in FL, MT, NC, and TX and one less in CA).
• The last column (SfΔ) compares the new maps in terms of fractional seat probabilities.
• The bottom line (TOTAL) is that the new maps favor Democrats slightly more (~1 seat), but still not quite proportionally (223.1 vs. 225), and that near balance obscures offsetting maps that are more favorable to Democrats (~7.59 seats shaded blue) and Republicans (~6.68 seats shaded red). While the MD and NC maps got fairer, the AZ, FL, TX, and TN maps and the IL, NJ, NM, and OR maps lean more heavily partisan in favor of Republicans and Democrats, respectively.

This analysis cannot account for the many changes in the electoral environment — midterm effects, 1/6, Roe v. Wade, gun control, etc. — that could affect relative turnout and shift the outcomes significantly.

Source: Dave’s Redistricting uses VEST data. We implement John Nagle’s method which uses fractional seat probabilities (whitepaper) and infers a seats–votes curve using proportional shift. I thank him for valuable comments on earlier versions of this post.