Summary: My analysis indicates that redistricting has given Republicans an advantage of about 1.2% in national popular vote margin compared with pre-2010. In a close national Congressional race, which we have this year, this translates to an advantage of 13 seats. The effect is more than I was expecting.
Before I update my House prediction, it is necessary to quantify two reader concerns: incumbency and redistricting. Today I quantify the net total effect of redistricting from 2010 to 2012, and give the results of analysis that sharpens my previous estimate of incumbency effects.
For the first time in many decades, Republicans have dominated the post-Census redrawing of Congressional districts, controlling twice as many districts as Democrats. Do the resulting changes lead to a structural advantage, i.e. will the Democrats have to win more than 50% of the two-party vote to achieve a near-tie in House seats?
Many of you are convinced that the resulting tilt to the playing field is large enough to definitively rule out a Speaker Pelosi in 2013 – a big thumb on the scale. Some commentators share your view, including Rasmussen Reports. However, Congress-watcher Stuart Rothenberg says the new maps are “a wash.” Indeed, this was an assumption in my previous prediction regarding House control. Who’s right, me and Stu – or you and Ras?
This summer Charlie Cook wrote:
The Cook Political Report rates 211 House seats as solid or likely Republican, compared with 171 as solid or likely Democratic. If the 24 toss-up races split evenly between the parties, Democrats would score a net gain of just a single seat. Even if Democrats held everything in their solid, likely, and lean columns and also won every toss-up, they would still need to take two-thirds (12 of 18) of the districts rated lean Republican to win a majority. That’s a pretty unlikely scenario, absent a strong wind at their backs.
This kind of fine-grained analysis takes a lot of knowledge, but is hard to reduce to a single quantitative insight. I set out to make an estimate of redistricting’s effect in units of popular opinion – reminiscent of the Popular Vote Meta-Margin for the Presidential race.
To make that estimate, a key concept is the Partisan Voting Index (PVI), developed in 1997 by Cook. The PVI is defined as how partisan a state or Congressional district leans compared to the country as a whole. For example, PVI = R+2 means that on average, a Republican candidate will typically get 2% more of the popular vote than the national average.
So on average, the PVI should be zero, right? No. Changing this average has been the goal of Republican-dominated state legislatures. If they can corral lots of Democrats into fewer districts, then it will take fewer Republicans to win the remaining districts.
Thanks to redistricting geeks on both sides of the aisle, I did not have to compile PVI’s. Republican activist David Lasdon helpfully provided me with a spreadsheet of post-redistricting PVI’s. He calculated it using Dave’s Redistricting App, a software tool created by computer scientist and Democratic activist David Bradlee to let individuals create their own Congressional districts. I also have numbers from the redistricting team at DailyKos Elections. These give very similar results. The graphs below come from Lasdon’s data.
I sorted the PVI’s and plotted histograms:
In this format, it is not easy to see what the net effect is. A better approach is the cumulative histogram:
Here, the vertical quantity is how many pre-redistricting Congressional districts were at least X% more Republican-leaning than the value on the horizontal X-axis. The most Republican-leaning district shown is R+10%, and about 120 districts are more Republican than that. At the right is D+10%; 320 seats are less Democratic-leaning, i.e. 115 seats are more Democratic-leaning.
Actual election outcomes are superimposed, converted to match the curve. Red asterisks show 2002-2010, which should have been predicted most strongly by the districting curve. The fit is not particularly good, though unfortunately we lack data at the crucial 217.5-vote point. Overall, it seems that voters lean slightly Democratic in their Congressional preference compared with their Presidential-based PVI. This was even true when Republicans had the majority (the points above the horizontal red line).
Blue asterisks show 1992-2000 and black circles show earlier periods, 1946-1990. The general cloud of points is fairly fat, so that while redistricting must be taken into account, it is just one of many competing factors. As I have written before, until 2012, an evenly split House popular vote would have led to an approximately evenly split House.
However, there is something we can answer: how much did the playing field change for the 2012 elections? In combination with what we know about pre-2012 outcomes, this would be a very useful quantity. We can do that by comparing the cumulative histograms:
The black curve shows data pre-redistricting, and red shows post-redistricting data. The curves are nearly overlapping…or are they? Let’s look closer. Zoom in to the middle 100 of the PVI’s:
The main point is this: this midrange is shifted to the left for the new districts. As a group, these middle 100 districts are somewhat more Republican-leaning than the middle 100 districts from 2008. They are not necessarily the same districts, but that does not matter for this level of analysis. In other words, at any given level of national partisanship, the number of districts that lean Republican is greater after redistricting.
Alternately, look at the shift in the vertical direction. The more people vote Republican on November 6th, the further right we will be on the axis, as each district’s PVI is overcome. The red curve is above the black curve, which means that the same national vote will yield more Republican seats. This is the structural advantage from redistricting. It is largest for GOP caucuses smaller than 254 seats – in the range where it matters.
How can we quantify the advantage in terms of national popular vote? That can be done using the horizontal distance between the curves – how far one curve would have to be shifted to align it with the other curve. The average shift over the range shown is R+0.62 +/- 0.06% (mean +/- SEM). In terms of seats, use the vertical distance to get a redistricting advantage of R+6.3+/-0.6 seats.
To get effective vote and seat margins, multiply these numbers by 2. Democrats will need to win a 1.24% larger margin to achieve the same number of seats they would have gotten in 2010. And for a given popular vote split, Republicans will have a larger House margin by 12.6 seats on average.
So: in terms of national popular vote margin, the Congressional 2012 new redistricting advantage is R+1.2+/-0.1%.
Finally, a brief note on incumbency. Although incumbents walk into the general election campaign with an advantage, as shown by Andrew Gelman, much of that advantage should already be captured by the generic Congressional ballot.
Previously I estimated the incumbency advantage this year, averaged over all districts, as R+1.3 +/-1.8%. This is based on 1996-2010 data. Based on 1946-2010 data it becomes R+1.3 +/- 0.4%. This is a second net potential advantage for Republicans.
The sum of these is R+1.8 +/- 1.0%. This is the amount by which Democrats must win the popular vote in order to have approximately even odds of winning control of the House of Representatives. I previously assumed an advantage of R+1.3+/1.0%. The next House prediction will use the new calculation.
I thank Nathan Persily and Steven Schultz for discussions. I also thank David Lasdon and Daily Kos Elections for their calculations. This kind of geekery is bipartisan. Lasdon seemed to think I was a political scientist, and wrote: “I look forward to being a footnote in a paper I’ll never read.”