1976 was a horrible year for Senate Republicans; adjusting for that fact makes a slight difference to my 2018 predictions.
Re-examining the results for my original model of Senate elections, it was hard to ignore how poorly the model fit the data for 1976. Here is a graph of the model’s predicted vote for Senate and the actual vote that shows what an “outlier” 1976 is. While Truman rallied Senate Democrats in 1948, even that event just hovers on the edge of the statistical “margin of error.” The Republicans’ failure in the 1976 election after Richard Nixon was forced from office stands truly alone compared to the rest of the postwar elections in my dataset.
If 1976 had been a normal presidential election year, the Republicans’ Senatorial prospects would have looked fairly rosy. Gerald Ford was running for re-election, real personal income was growing at two percent, and the Democrats were defending seats won by a (two-party) margin of 56-44 at the height of the anti-war and anti-Nixon fervor. That generally pro-Republican climate predicts the GOP should have won nearly 52 percent of the popular vote for Senate.
But, of course, the 1976 election was anything but normal. It was the first presidential election after the Watergate scandals had forced Richard Nixon from office in disgrace. Rather than winning the popular vote by the predicted four-point margin, the Republicans could muster only the same share of the vote they won back in 1970, 44 percent. Though a number of seats changed hands, at the end of the day the Democrats held the same 61-seat Senate majority they did before the 1976 election.
I can adjust statistically for the anomalous 1976 election by adding a “dummy” variable to my model that is one in 1976 and zero otherwise. Adjusting for 1976 radically improves all aspects of my model. Its predictive power as measured by adjusted R-squared rises from 0.43 to 0.56, and all the coefficients are more precisely estimated.
Adding this dummy variable implicitly treats Gerald Ford as different from other Presidents running for re-election. Ford was apparently so compromised by Watergate that his presence at the top of the ticket did not generate the kind of support his fellow Republican candidates for Senate might have expected. With the 1976 adjustment, the overall effect of Presidential candidacies rises from 2.5 to 3.1 percent, suggesting Ford’s performance was suppressing the estimate for other Presidencies.
Adjusting for 1976 also increases the compensating effect (“regression-toward-the-mean”) of the prior vote for a Senatorial class. With the suppressing effect of 1976 removed, I now estimate that the Democrats’ lopsided Senate victory in 2012 should be worth about 2.3 percent to the Republicans this November, compared to the 1.9 percent figure I presented earlier with no adjustment for 1976.
For comparison to the chart above, here are the predicted and actual values for the model adjusted for 1976:
Including a dummy variable for 1976 sets its residual to zero and places its predicted value precisely on the line. The largest positive outliers are now 1978 and four Presidential years, Truman in 1948, Johnson/Humphrey in 1968, and the two elections held while Barack Obama was President, 2012 and 2016.
The effects of this modification on the predictions in my earlier article are quite modest. Without adjusting for 1976, I predicted the Republicans will win 48.1 percent of the popular vote if Trump’s approval rating is at forty and real disposable personal income rises two percent. With the adjustment that figure rises to 48.4 percent.
The overall conclusion remains that no likely combination of factors predicts that the Republicans will win a majority of the popular vote for Senate this fall.