In the previous article I showed that Democrats must win at least 53 percent of the national two-party vote for Congress in order to retake control of the House of Representatives. That higher hurdle to success reflects the combined effects of more extensive partisan gerrymandering by Republican state governments and the tendency of Democrats to live in densely-populated urban districts. These factors make Democratic votes for the House less “efficient” than Republican votes when it comes to determining which party controls the chamber.
So what combination of political and economic factors might result in a Democratic vote of 53 percent? Political scientists have presented a number of models for mid-term elections over the years. In an early paper, Edward Tufte showed that presidential approval and short-term changes in personal economic conditions both influenced support for the incumbent using the small sample of mid-term elections he had available at the time. I find little support for an economic effect, but presidential job approval does play an important role.
I have analyzed both all Congressional elections and off-year elections separately. The overall results are quite similar. I am basing the conclusions below on the data for the seventeen off-year elections in my sample from 1950 to 2014. Rather than treat the parties symmetrically and examine support for the President’s party as I did for the Senate, I am focused this time specifically on factors influencing support for the Democrats in off-year elections since their vote is what matters to this analysis. It turns out just three variables account for over 90 percent of the variance in the Democratic vote for the House:
As always, the dependent variable is measured as a logit. Values above zero are associated with probabilities above 0.5; negative values represent probabilities below 0.5. So the positive constant term indicates that the Democrats had an advantage over the period, but the coefficient for the dummy variable representing elections after 1992 is about equal in size and opposite in value. That pattern corresponds to what we saw in the last article where Democrats had a seat advantage in the House until 1994 that vanished for two decades and has now turned significantly negative.
The other two variables capture the “referendum” aspect of off-year elections. The Democrats do worse on average when one of their partisans occupies the White House. However rising job approval ratings do translate into more support at the polls in the off year. (The approval variable is coded positively for Democrats and negatively for Republicans. If separate terms are included for Democratic and Republican presidents, the estimated coefficients are nearly identical in size but opposite in sign. The coding I used imposes the constraint that changes in Presidential approval ratings have the same sized effect for both parties. The job approval data comes from Gallup and is based on averages of their polls near the election.)
I tried a variety of measures of economic conditions, specifically changes in real per capita disposable personal income, and none of them showed any additional effect. I included a test of the “myopic” voter theory using only the change in income comparing the third and second quarters of the election year. That fared no better than an approach with a longer time horizon, the growth rate over the past twelve months. Thus there is no term in my model for economic conditions.
Since we have a Republican president, my estimates are based on the sum of the constant term and the term for elections after 1992. If I plot the model’s predictions against President Trump’s potential approval ratings, I get this relationship:
If the President’s job approval rating falls below 32 percent, the model predicts the Democrats would win the 53.2 percent of the national House vote that we saw in the last article is required to obtain a majority of the seats in the chamber. The last three Gallup polls reported Trump’s job approval at 38 or 39 percent.
An approval rating below thirty is historically very unlikely. Richard Nixon in 1974 and George W. Bush in 2008 had ratings in the mid-twenties. Jimmy Carter in 1978, George H. W. Bush in 1992, and his son in 2006 received job approval scores in the mid-thirties. Of course, all of these incumbents had much higher ratings when they took office than did Donald Trump.
The average decline in Presidential job approval between Inauguration Day and the first subsequent off-year election has been a bit under nine points. That would take Trump’s score down toward the mid-thirties. However because he started at just 45 percent approval when inaugurated, he may not experience the same decline as did presidents who started from a higher rating. For instance, it seems unlikely that Trump will experience a decline on the order of 23 points like Barack Obama did going into the 2010 midterm. In fact, the table suggests the public treats Republican and Democratic presidents quite differently. The Democrats all posted double-digit declines in job approval by the first mid-term election; none of the Republicans lost more than nine points over the same period, and approval for both Bush presidencies actually increased.