To the extent that gerrymandering hurts Democrats in their attempts to reclaim the House and promotes extremism, it is merely a symptom, not the disease. The disease itself has three components. The first is noted above: Partisan districts have tended to elect fewer and fewer members from the opposing party. Because there are more Heavily Partisan Districts that lean Republican than lean Democrat, this disproportionately affects Democrats. Note that this tendency is longstanding and more or less constant: With the exception of 1996 and 1998, there have always been between 29 and 36 more Highly Partisan Districts that lean Republican than lean Democrat.

The second part of the problem is related: We are polarizing the seats. Look back to the second chart. There were two reasons I broke out the McCain and Romney PVIs. First, it allowed for a more apples-and-apples comparison to 2008 and 2010. But second, and more importantly for our purposes now, we see that the post-2010 lines themselves became more polarized after the result of the 2012 elections.

Under the present lines, using the McCain/Obama results, there were 123 seats that were R+10 or more and 110 seats that were D+10 or more. If we take the exact same lines, but use the Romney/Obama presidential results as our yardstick instead, we find 141 seats that are R+10 or more, and 121 seats that are D+10 or more. Looking at our broader set of Heavily Partisan Districts, we see eight additional heavily Republican districts and two additional heavily Democratic districts come into being just from the country becoming more polarized from 2008 to 2012. In a very odd way, we’re gerrymandering ourselves.

The third problem is specific to Democrats. Their coalition has become increasingly geographically concentrated, or pushed into majority-minority districts.