Gerrymandering and House elections

While most of the attention focuses on the presidential race, over at the Princeton Election Consortium Sam Wang has an interesting analysis of the data on the relationship of the national vote for the two major parties in each biennial election to the number of seats that they ended up holding in the House of Representatives.

In the figure the x-axis represents the popular vote margin for the Democrats over the Republicans and the y-axis represents the excess of House seats. The dots represent the results for each biennial election since 1946, with the 2010 results specifically indicated. The grey area represent the range of the data and you can visually estimate that a trend line would pass close to the origin (0,0), which means that if the two parties tie in the national vote, they should get an equal, or close to equal, number of seats.

Unlike the case for the presidency, where because of the electoral college system the winner need not be the person who wins the majority of the national vote, you would expect that control of the House would depend upon it. However, the black straight line represents what Wang thinks is the current relationship between these two numbers and the red line represents his range of predictions for this November’s election based on the current polls. You see that these two lines do not pass through the origin. There is now a built-in 2.5% bias towards Republicans, meaning that the Democrats need to get 2.5% more of the national vote than the Republicans just to expect to break even in the number of House seats.

Wang puts this down to gerrymandering of house seat districts following the Republican election landslide in 2010 that put Republicans in control of the redistricting process in the majority of states. He says that this will be a feature for the next decade, until redistricting is again done following the 2020 census.


  1. Pierce R. Butler says

    I live in the most gerrymandered – or at least most etiolated and weirdly-shaped – district in Florida (# 5, currently held by Corinne Brown).

    The line-drawing – personally seen to by the state legislature – was approved by the feds, apparently on the grounds that this district will almost certainly elect a black Representative from NE Fla.

    From here, it looks more like the lines were drawn to make sure there wouldn’t be two black Representatives from NE Fla.

  2. Eric Ressner says

    I think I must be missing something. Why is the black like (and correspondingly the prediction for 2012 represented by the red line) not somethink akin to a best fit to the data points?

    Is it that the most recent elections are all toward the bottom of the gray area, illustrating the drift toward (R)’s built-in advantage? If that’s the case, it would have been nice to see, say, all six of the most recent biennial elections labeled.

    If that’s not the explanation, what is?

  3. Eric Ressner says

    Sorry, that should have been “black line”, not “black like”. Why is clicking on “Submit Comment” a cure for blindness?

  4. fastlane says

    There is no reason (other than politics) that each state couldn’t simply start with a grid, based on the number of reps for the state, then use a computer algorithm to redraw the grid in a way that makes it as close as possible for each district to represent an equal portion of the state, based on population.

    Take as much bias out of the system as possible and see if we could get closer to an actual representation.

  5. Mano Singham says

    The black line is his best estimate, based on the current electoral map as a result of the latest redistricting of the relationship between the two numbers following the 2010 census.

    The red line is based on the current polls across the nation and represent the likely range of outcomes for the 2012 election, so it is a subset of the black line.

  6. Skip White says

    The district in which I live (in Pennsylvania) was redrawn following the 2010 census, and I now have no idea what the area of the new district includes, or who is running. One of the more blatant examples of gerrymandering in my state is the redrawing of the 11th congressional district. It previously included a good swath of northeastern PA. Now, it includes Hazleton, in NE PA as before, but also includes a good chunk of Cumberland County, about 80 miles southwest of Hazleton. In order to accomplish this, a thin connecting line was drawn all the way down.

  7. brucegee1962 says

    I live in Virginia, where a group of poli-sci students did exactly that before the last round of redistricting. The politicians all had a hearty laugh before tossing that plan into the trash can and mashing the districts around like silly putty.

    Interestingly, the plan they came up with wasn’t really designed to primarily help Republicans. No, the group they really wanted to protect were the incumbents of both parties — that is, the ones who were drawing the map. The student-drawn maps would have put two incumbents in the same district, which Just Isn’t Done.

  8. baal says

    Gerrymandering is one (of a long list) where I’ve been annoyed with the SCOTUS. They are ok with it – largely on an argument that goes something like, “Duh, politics is political so gerrymandering is politics in action. Don’t like it? Get more votes!!!(eleventy)!1!!”

    I like the idea of having a computer draw the lines. The fight would move to the optimization parameters but that at least comes across more honestly. “Hrm, our side wants no more than 12% black per voting district” vs “12% blacks should be able to hit 12% of seats, redraw until that’s possible”. (Just straight pop would probably wind up closer to the first than the second quote since minorities are concentrated and every black over 50%+1 would wind up not getting represented. I’ve dropped a lot of nuance but the basic outcome is the same)

  9. Nick Gotts (formerly KG) says

    That the US political system gives responsibility for redistricting to directly interested parties is completely gobsmacking to outsiders. Just about every other country even claiming to be democratic has what is at least supposed to be an independent and impartial body to do that.

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