Despite the increased ingenuity and sophistication of gerrymanders, numerous constraints and obstacles impede using redistricting as an "incumbency protection" plan. The requirements of equal population, compactness, and contiguity reduce the scope of such manipulation. Because many states have relatively few districts, gerrymanders often lack the flexibility to created distorted districting plans.
The paper also notes that partisan gerrymanders -- along the lines of what Republicans in Texas led by Tom DeLay did in the early 200s or what Illinois Democrats did in 2010 -- don't actually make less competitive districts but create more. By spreading out the majority party as thinly as possible in as many districts as possible, the minority party's seats tend to be made safer while the remainder of the districts are made more competitive in hopes of maximizing gains. "Partisan gerrymandering leads to more competitive districts than noncompetitive districts and has an ambiguous effect on polarization," write Nolan McCarty, Keith T. Poole and Howard Rosenthal in the paper.
And, to their point, the death of the middle in the House has been accompanied by a similar shrinkage of the so-called "moderates" in the Senate -- a body unaffected by redistricting. Here's the Senate version of that same chart.
So, if redistricting didn't kill off the middle, what did? Theories abound -- and, in truth, each likely played some role. (Redistricting also played a role, although not as big of one as most people seem to think.)
1. Self sorting: We have, over the past several decades, grown much more likely to live around like-minded people. That like-mindedness extends beyond whether or not you cut your grass every weekend to deeply-held values and political beliefs. The country is simply better sorted today than it was in, say, the 1980s. That means that the people who get elected to serve in Congress -- and even the Senate -- tend to represent constituent views that are clumped either on the far left or far right of the ideological spectrum. In other words, we get the Congress we want, ideologically speaking. (John Sides, who runs the Monkey Cage blog, takes issue with the power attributed to self-sorting.)
2. The South's move from D to R: In 1982, half of the 22 Senators who represented the states of the Confederacy were Democrats. Thirty years, later only five were left; and that number drops to two -- Mary Landrieu in Louisiana and Mark Pryor in Arkansas -- if you restrict it to the deep south. (Virginia has two Democratic Senators while Kay Hagan represents North Carolina.) While the changeover of the South from Democrat to Republican en masse in the House came in 1994, it was a more gradual process in the Senate -- thanks in part to the six year terms of Senators. In 2004, however, Democrats watched North Carolina, Georgia, South Carolina, Florida and Louisiana all turn from D to R.
3. The parties are farther apart than ever before: The two major parties agree on virtually nothing at this point. Need proof? Go back and look at what Congress has accomplished (or not) over the past few years. The parties' agenda and priorities are so far apart as to render any sort of crossover voting -- even in the Senate -- mostly moot. Again, McCarty, Poole and Rosenthal: "The secular increase in polarization is not primarily a phenomenon of how voters are sorted into districts. It is mainly the consequence of the different ways Democrats and Republicans would represent the same districts."
N0 one thing killed the ideological middle in Congress. It died a death of 1,000 cuts. But, that doesn't change the fact that it's dead -- and with little hope of being revived, at least any time soon.