In a prior series of posts, I constructed a “simple model of Senate elections” using national data across elections. This helped identify some key factors that influence the overall vote for Senators but provided no insight on the results in specific states. In this post I develop another “simple” model that is designed to predict the voting outcome based on two factors, a state’s partisanship as measured by support for President Trump, and the net favorability of the incumbent Senator. I estimated the model using data from the 2016 and 2018 elections. The results appear here and are best summarized in this chart:
The lines portray how the vote for an incumbent Senate Democrat improves as her net favorability grows. The top line represents the result for a Senator from a strongly pro-Democratic state, one where only 40% of the state’s voters approve of the President. Even a Democratic incumbent with a net favorability of zero is predicted to win nearly 55% of the vote in this state and hold the seat. In contrast, a Democratic incumbent in a pro-Trump state like Doug Jones in Alabama fails to win 50% of the vote even if he is unusually popular despite the party mismatch. Overall the Republicans hold a slight advantage. The model predicts that in a state where support for Trump is 50-50, the purple line, only a Democratic incumbent with at least a +8 favorability has a chance of holding the seat.
We can apply the results of this model to the 2020 Senate elections. We only have available the current measures for Trump support and candidate favorability, so we obviously cannot predict how things will stand a year from now. For the estimates below, I have used the most recent Trump approval rating and Senate incumbent favorability ratings as reported by Morning Consult. The President’s score is from the month ending September 1st; the Senators’ ratings are averages over the third quarter, July-September, 2019.
The highlighted rows at the top of the table correspond to incumbent Senators whose predicted vote is below fifty percent.The top and bottom spots on the list are held by Democrats. The most vulnerable incumbent is Doug Jones’s whose slight positive favorability rating of +5 is nowhere near large enough to overcome Alabama’s warm feelings for Donald Trump.
Jones is followed by the four most commonly discussed vulnerable Republicans — Susan Collins of Maine, Cory Gardner of Colorado, Joni Ernst of Iowa, and Thom Tillis of North Carolina. Martha McSally would hold her Arizona seat by the slimmest of margins. Majority Leader Mitch McConnell is lucky to represent solidly pro-Trump Kentucky or else his dismal favorability score might lead to his defeat.
It’s anyone’s guess what Donald Trump’s approval rating might be come the election next November, though his score has remained remarkably persistent in the face of events. Using the averages at FiveThirtyEight, we see his low point came in the summer of 2017 when he fell to 37%. Over that winter and into the spring of 2018 his approval rating improved to about 42% where it has largely remained. There was a dip in his popularity during the government shutdown, and another now as the impeachment inquiry expands. Given the observed variation in his popularity since the Inauguration, Trump’s approval rating might move up or down by three or four points over the course of the next year. A four-point movement would represent a ten-percent change from his current rating of 41%. The chart below shows how each Senator’s predicted vote would change given a ten percent increase or decrease in Trump’s approval rating in each state.
The four Senators at the top of the list in the darker grey area are predicted to lose their seats even if Trump’s approval rating were to improve by ten percent. The next three Senators survive their re-election bids if Trump’s approval runs about where it is today or improves by November, 2020. However a ten-percent decline in Trump’s approval threatens the seats of Thom Tillis, Martha McSally, and even Mitch McConnell.
Right now the Republicans control the Senate by a 53-47 margin, plus the tie-breaking vote of the Vice President. Assuming a Democratic victory in the Presidential election next fall, the Democrats need to flip at least three seats, while losing Alabama back to the Republicans. Maine, Colorado, and Iowa look promising for the Democrats and North Carolina and Arizona are both tightly contested.
Four Republican seats have vacancies. In Georgia a special election will be conducted in 2020 alongside the regular election to fill the seat that Johnny Isakson will leave at the end of this year. Three other Republican-held seats will also be vacant in 2020. My model predicts the Republicans will hold all these seats with Georgia the most competitive. (To construct these estimates I impute a favorability score for a “normal” Democrat by regressing net favorability on Trump support to account for the partisanship component of favorability.)
In strong Republican states like Wyoming and Tennessee, we see support for Trump running in the mid-fifties. In states like these, a Democratic challenger would do well to card a favorability score better than -15. The states where the Democrat might have some chance are Georgia and Kansas, where support for Trump splits evenly, but still the Democrats are predicted to lose those elections by three or four points.
