The Parties Are Not Symmetrical

I wanted to take note of Sean Trende’s interesting analysis of the prospects for the midterm elections at RealClearPolitics. The graphic above, illustrating Joe Biden’s vote share by Congressional district in the 2020 general election, was “sampled” from that post.

The vertical red line illustrates Mr. Biden’s percentage of votes received nationally. Note the “significant valley right at the national average”. What that tells us is that in the districts Biden won he won by considerably more than his national average and in the districts he lost he lost by considerably more. Shorter: the districts are polarized. That’s a consequence of gerrymandering, both by Republicans and Democrats.

Here’s a telling quote:

But one consequence in the near-elimination of swing districts is that there are substantial “levies” on either side of the valley. The levy on the Democrats’ side is particularly steep. So in a universe where Republicans win the popular vote by four points, sweeping all of the districts that Biden won with 54% of the vote or less, the levy would break and the Republican majority would jump from 232 seats to 245 seats. Winning by six points leads to smaller gains as we work down the other side of the levy, leading to a majority of 252 seats. Of course, these are approximations; Republicans would probably win some seats beyond Biden +10 and lose some where Biden performed worse in this scenario. This is just to illustrate how the playing field works.

There’s a similar “levy” on the Republican side as well, although the slope is much gentler. Recall that sweeping the districts that were 52% Biden or less yields 224 seats, while sweeping those that were 51% Biden or less yields 219 seats. Winning the 50%-Biden-or-less seats would give Republicans 206 seats. If the cutpoint were 49% Biden or less, Republicans would win 198 seats. Note the asymmetry. Even with a cutpoint of 47% Biden or less, Republicans would get 191 seats to the Democrats’ 244. In other words, because of the different placements of their levies, Republicans could lose the popular vote by around ten points and have more seats than Democrats would have if Republicans won the popular vote by six points.

I found that an interesting analysis.