I used a relatively simple model to generate the graph shown in the accompanying article. I ran weighted least-squares regressions against the logarithm of the number of days since the Inauguration and the usual array of dummy variables to account for differences in polling methods and “house effects.” Here are the results:
The use of the base 10 logarithm shows that Trump’s approval is estimated to have fallen by a bit over five percent between day ten of his term (log(10)=1) and day one hundred (log(100)=2). I use the percent approval rating in this regression to make the dummy variable coefficients more meaningful. A regression using the logit of the approval rating shows identical results.
These results show that polls conducted over the Internet or limited to registered voters tend to report a job approval figure about 1.5 percent higher than the consensus. The “house effects” variables indicate that polls conducted by YouGov/Economist average just under one percentage point less favorable toward Trump than the consensus. Job approval ratings from polls taken by SurveyMonkey, Politico/Morning Consult, and Rasmussen run from two to five percent higher than the consensus. Gallup, the most frequent pollster, does not deviate from the consensus.
In the preceding two articles I analysed the prospects that the Democrats could regain control of the House of Representatives. The combined effects of gerrymandering and partisan self-selection geographically mean the Democrats face a substantial burden to regain a majority in the House. Only if the Democrats can win at least 53 percent of the national popular vote for Congress do have they have a decent chance to attain a majority.
The success of the Democrats at the polls in mid-term years depends primarily on two things, whether their party holds the White House, and the President’s job approval rating. For the Democrats to reach that 53 percent floor needed to retake the House, President Trump’s job approval rating would have to fall to 32 percent or less.
While there have been occasional polls which put Trump’s approval in the low thirties, they have generally been outliers. I have taken all the polling data available at Huffington Post Pollster since the Inauguration and estimated simple trends that look like this:
The bottom line represents the trend for all adults interviewed in person. This is the sample used by Gallup, the most prolific pollster in this group. Registered voters, or citizens interviewed over the Internet, show a slight pro-Trump bias of about one and a half points. The top line shows the estimate for registered voters interviewed via the Internet. (Polls of so-called “likely voters” were not significantly different from all polls of registered voters.)
If we take the plunge and extrapolate these trends to the time of next year’s mid-term election (about 650 days in office), Trump’s job approval rating falls only two more percentage points, ranging from 34.6 to 37.9 percent depending on which trend is chosen. That is still short of the 32 percent threshold I estimated in the preceding two articles.
The results for registered voters should give the Democrats more pause. The citizens eligible to vote have a more positive view of the President than those who remain unregistered.
Now that all the gnashing of teeth has ended after the Republicans managed to hold on to the Georgia Sixth, perhaps we can step back and take a more systematic look at the Democrats’ prospects in 2018. Democrats will likely not make any gains in the Senate since the Republicans have only eight seats at-risk compared to twenty-three Democrats and both independents, Maine’s Angus King and Vermont’s Bernie Sanders. That leaves the House as the only target.
There are two steps involved in answering this question. The first is to use our historical experience with House elections to examine how votes are translated into seats. With that information we can estimate the proportion of the two-party House vote that the Democrats need to win to take back the House in 2018.
As I wrote back in 2012, a combination of geographic clustering by party and good old partisan gerrymandering has created a “Republican bulwark” in the House since the last redistricting after the 2010 Census. That means that the Democrats will need to win more than a majority of the popular vote for Congress if they intend to win a majority of House seats.
I have refined this simple seats and votes model in two ways. First, I let the “swing ratio” vary between two historical periods, 1940-1992 and 1994-2016. Empirically the effects of voting “swings” on seat “swings” is significantly smaller in the more recent period. As Tufte argues in his classic paper on the seats/votes relationship, a decline in the swing ratio indicates an increase in the proportion of “safe” seats. As fewer and fewer seats have vote shares around fifty percent, there are consequently fewer that can be “flipped” by an equivalent shift in voters’ preferences.
I also use the results for the 2014 and 2016 elections to more sharply estimate the effect since 2010. If we calculate the popular vote share required for the Democrats to win half the seats in the House, they would need to secure a bit over 53 percent of the (two-party) votes cast.
That brings us to the second question, what are the chances that the Democrats could win 53 percent of the Congressional vote in 2018? Answering that question deserves an article unto itself.