The Effect of Redistricting on House Elections, Revisited

Oct 15 '12

The Cook Political Report has come out with its “PVI” numbers of each of the new districts, which are measures of district partisanship based on the presidential vote.  They use this measure to argue for an important redistricting effect this cycle in favor of Republicans.  David Wasserman has the details:

Unfortunately for Democrats, this year’s index tells a dire story of what can happen when a party suffers an ugly election cycle right before redistricting. Democrats…lost…more than 680 state legislative seats – empowering Republicans to draw ten-year maps in four times as many districts as Democrats. As a result, thanks to effective GOP cartography, the number of “strong” Republican seats has jumped from 182 to 190 and the number of “strong” Democratic seats has fallen from 150 to 146. Meanwhile, the number of “swing” seats has fallen below 100 for the first time, from 104 to 99. (emphasis in original)

The Cook report is a venerable organization whose analysis is rightly respected, and I have no doubt that their presidential vote numbers by district are the most accurate available.  But the notion of big redistricting effects this year is Zombie Politics, and the poor corpse should be put to rest.

First, contrary to the quote above, a party that aims to gain seats through redistricting wants fewer “strong” seats, not more.  Since one only needs 50%+1 votes to win (in a two-party race), every vote beyond that threshold is “wasted” in the sense that it could be used to bolster the party’s chances in some other district.  More strong seats means bigger winning margins, which means more wasted votes.

Second, the idea that the 2010 elections dramatically increased Republican opportunities to gerrymander presumes they can actually control the process in the states they won.  This is not necessarily the case.  Here’s what we said on this subject a couple weeks ago:

[A] redistricting party doesn’t have free rein to do as it pleases.  Districts must be equal in population (the allowed deviation between districts is vanishingly small), the Voting Rights Act constrains activity in many states, and some states also have specific rules that limit the options.  (Along these lines, Florida voters recently imposed boundaries on what the legislature could draw, and Texas’s lines ended up being drawn by the courts.)  Moreover, a party’s desire for more districts can often conflict with its own incumbents’ desire for safe reelection.  And a redistricting party must work with the state it has, not the one it wants.  Republicans and Democrats may not live in close enough quarters to permit the sort of seat-maximizing gerrymander a party would otherwise want.

Finally, counting districts as Democratic or Republican using only the presidential vote assumes everyone who votes for Obama will vote Democratic for Congress, and ditto for Romney and the Republicans.  But even in today’s polarized environment, incumbency matters.  A moderately Democratic PVI with a Republican incumbent is probably a Republican win.  What happens when we pair redistricting with incumbency?  A close look at the Cook Report’s 25 incumbents most hurt by redistricting reveals that almost half of them (11) are Republicans.  And the Cook predictions for those races split evenly between the parties at 9 a piece, with the rest as toss-ups.

That’s why we feel the best approach incorporates both district partisanship and incumbency, and uses past election results as a guide for how important each is likely to be.  When we ran those numbers, we found redistricting was a wash.  We also found in the same analysis that Democrats would get a much more substantial seat gain if there were no incumbents running and all seats were open.  The structural advantage for Republicans this year stems from incumbency, not redistricting.