Three more Republican House Members announced they would not seek re-election this week, bringing the total number of retiring Republican Members to 34 according to the New York Times. That figure compares to 16 Democrats, for a net Republican difference of +18. We have to look back to the Democratic landslide in 1958 to see a mid-term with double-digit net Republican retirements. For Democrats, only in 1938 and 1978 did the number of their retirements exceed Republican retirements by ten or more.
This increase of three net Republican retirements raises the predicted Democratic seat swing to 41 using the relationship depicted in the previous article.
I have shown in earlier postings that the relationship between seats and votes that advantaged Democrats in the years after World War II moved steadily in the Republicans’ direction beginning in 1980 and, with the help of gerrymandering, became even more favorable for the Republicans after 2010. That might temper our belief in a prediction for an election being held in 2018. First, the 2018 retirement margin of -18 is close to the observed maximum of -21 in 1958. Perhaps the 1958 election is an “outlier” and without it the relationship is less steep than we observe. However slope and intercept coefficients estimated with 1958 excluded are numerically nearly identical to those estimated with that year included. So it’s unlikely that the historical model is radically overestimating the likely result next fall.
Another test is to let the relationship differ before and after 1992 to see whether the structural changes that we observe in the seats/votes relationship in the current era appear in the relationship for retirements and seat swings. Once again, allowing the coefficients to differ before and after 1992 showed no measurable statistical difference. While the effects of gerrymandering and partisan self-segregation may make the House less vulnerable to “waves” of Democratic support, there is no evidence for that thesis looking at retirements as a predictor of seat outcomes.
These estimates have a lot of uncertainty attached. The standard error of estimate is about 28 seats. That means there is about a two-thirds chance that the actual swing will be somewhere between 13 and 67 seats. Since the Democrats need a swing of at least 24 seats to win control of the House, even a retirement margin of 18 is not enough to ensure a change in party control.
The regression model taking President’s partisanship into account is a bit more conservative; it predicts a swing of 37 seats.
Update (2/26/18) – One more Republican has announced he is leaving the House, along with one more Democrat. The net difference remains at +18.