Democrats have won, on average, about eight fewer seats in each election since 2010 than we would expect given their popular vote. The surge in Democratic votes this year might have cut that deficit down to two, but it is more likely there was no effect at all.
Before the November election, some commentators argued that a surge in turnout could negate the effects of Republican gerrymandering after 2010. Of course, this argument only makes sense if there were a larger increase in Democratic turnout than Republican turnout. A proportional increase for both parties would leave the seat results unchanged.
It is certainly true that the Democratic vote for the House of Representatives was considerably greater in 2018 than it was in 2014. In fact, Democrats cast nearly as many votes this month as they did for Hillary Clinton two years ago. Compared to the 2014 midterm, the Democrats increased their vote by over fifty percent. Republicans also turned out in higher numbers, recording a vote for House candidates some 23 percent above their 2014 totals. (Figures for 2018 from Dave Wasserman of Cook Political Report.)
To test this, I added a term for the 2018 election to my standard model of seats and votes described here and here. I use the “logits” of Democratic seats and votes won with “dummy variables” to represent reapportionment periods. The basic model, with 2018 included, produces this chart showing the number of Democratic seats won or lost compared to what we would expect based on the national popular vote won by that party. Some periods, like 2002-2010, show no significant excess gains or losses. Others like 1942-1950 and 2012-2018 show substantial effects.In the five elections beginning in 1942, Democrats routinely won nearly nine more House seats than their popular vote would predict. Republicans picked up a number of state legislatures in the 1952 election and erased this deficit for the decade to follow. From 1962 through 1990, Democrats were again advantaged, but by a diminishing margin over time. The elections fought between 1992 and 2010 showed no systematic bias for either party After the 2010 Census and the “shellacking” of Democrats in both national and state elections that year, Republicans were able to draw district maps that gave their party just short of eight “excess” seats in the House.
By adding another variable to represent just the 2018 election, it does indicate a diminished effect compared to the 2012-2018 average. However this effect fails to reach any conventional level of statistical significance (t = 1.07).
One other question we might ask is what the 2018 outcome would have been had the neutral results for 1992-2010 continued on into elections held since the 2010 Census. While the chart above shows that Democrats lost on average about eight seats to gerrymandering beginning in 2012, the estimated effect for this past election is just short of fourteen seats, the result of the Democrats’ substantial victory in the popular vote.