The Effects of Reapportionment

The reapportionment process following the 2010 Census transferred twelve Congressional districts from ten largely Northern and Midwestern states to eight largely Southern and Western ones.  The largest beneficiary was Texas which gained four seats and increased its representation in Congress to 36 Members.  Florida gained two seats, and Arizona, Georgia, Nevada, South Carolina, Utah and Washington added one seat each.  New York and Ohio both lost two seats, while Illinois, Iowa, Louisiana, Massachusetts, Michigan, Missouri, New Jersey, and Pennsylvania lost one apiece.

While all states must redistrict every ten years, the effects are greatest in states gaining or losing representation in Congress.  Popular candidates from the same party in nearby districts can be pitted against each other in the primaries after their two seats are reduced to one.  New seats provide opportunities for creative cartographers to draw districts that not only command a majority for their parties today but over the next ten years as well.

Over the next few postings I will be exploring the question of why the Democrats fared so poorly in the Congressional elections.  I begin today not with the issue of gerrymandering, but the more fundamental question of whether reapportionment itself might have changed the partisan balance of the House.  Adding four seats to a state like Texas whose Congressional delegation was already 23-9 in favor of the Republicans should on average have given that party three of those four seats. Removing a district from Massachusetts where all the Members were Democrats means the sure loss of a Democratic seat.

How many seats did reapportionment affect?
On the face of it, this question should be easy to answer.  Since twelve seats were moved, the largest effect it could have is twelve, and only then if every seat lost belonged to one party and every new seat was won by the other party.  Since both Democratic New York and Republican Louisiana lost seats in 2012, and Republican Texas and Democratic Washington gained seats, calculating the actual effects of reapportionment requires a bit more effort.

Start again with Texas with its four new seats and a Congressional delegation that had a 23-9 margin in favor of the Republicans.  If we draw the new districts proportionately to maintain the current partisan balance, we would expect the 2012 Texas delegation to include, on average, 2.9 (=4*23/(23+9)) new Republicans and 1.1 (=4*9/(23+9)) new Democrats. Using just the 2010 results overstates the extent of Republican sentiments because of the size of the 2010 Republican landslide.  I used instead the average partisan split in both the 2008 and 2010 Congressional delegations to estimate the division in 2012.  For Texas, this increases the Democrats’ estimate a bit from 1.1 to 1.3 when we base that figure on both the 2008 and the 2010 results. The top half of the table includes the data for states that gained a seat in 2012; those that lost a seat appear in the bottom half.

In 2008 Republicans held 55 of the 99 Congressional seats in states that added a district in 2012.  In the 2010 Republican landslide they pushed their advantage up to 68 seats. Depending on the outcome in the Arizona 2nd, Republicans will hold 72 or 73 of the seats in these eight states when Congress convenes in January.  Is this a strong showing for the Republicans across these states?  How might we tell?

The last three columns provide an answer to that question.  If we allocate each state’s new seat(s) in proportion to the average division in that state’s Congressional delegation in 2008 and 2010, the Democrats should have gained 4.6 of the 12 new seats and the Republicans 7.4. The Democrats actually won either seven or eight seats in these states depending on Arizona. While the Democrats added not one of the four new seats in Texas, they did pick up four seats in Florida, one each in Nevada and Washington, and either one or two seats in Arizona.

Democrats also fared slightly better than expected in the states that lost a Congressional seat in 2012.  Applying the same proportional model predicts that the Democrats should have lost 6.7 seats.  Their strong showing in Illinois held their losses to just five.

Overall, then, it seems like the Democrats not only survived the process of reapportionment in the states most immediately affected, they actually carried four or five more seats than we might have expected.  Strong performances in Florida and Illinois were largely responsible for this result.

Reapportionment and the 2012 House Results
In the opening posting on the 2012 Congressional elections, I estimated that the Democrats now face a ten-seat disadvantage in the House based on that party’s share of the popular vote.  The method above suggests that we can attribute the loss of perhaps two of these seats to the effects of reapportionment itself. Using the method of extrapolating the pro-Republican trend from 1960-2010 results in an estimate of four seats.  That leaves some six to eight seats that constitute the Republican “bulwark” still to be explained.