Democrats would lead by nearly fourteen points in “generic” Congressional ballot polls next November if the trends seen since Trump took office continue.
I have written earlier about how methodological differences among pollsters can lead to significantly different results. In my analyses of Presidential approval I showed how Donald Trump’s approval ratings varied depending on the choice of sample to interview and the interviewing method chosen. In this piece I apply the same approach to the so-called “generic” ballot question, typically “If the elections for Congress were being held today, which party’s candidate would you vote for in your Congressional district?” Some pollsters mention the Democrats and Republicans in this question, others leave it more open-ended like the example I just gave.
I have focused on the net difference in support for generic Democratic and Republican candidates. This ranges from a value of -4 (Republican support being four points greater than Democratic support) to a high of +18 in the Democrats’ direction. Here is a simple time plot showing how support for the Democrats on this question has grown while Trump has held office.
The Democrats held a small lead of just under four points on the day Trump took office. Since then the Democrats’ lead has slowly increased to an average of eight points.
What’s surprising about these data is that they do not show the usual methodological differences we see in the presidential series. Here are a few regression experiments using my standard array of predictors.
Choice of polling method has no systematic relationship with Democratic support on the generic ballot question. In contrast, Trump’s job-approval ratings run one to two points higher in polls taken over the Internet. Another striking difference is the greater level of support found for Democrats in polls of registered or “likely” voters. Again, the job-approval polls show an opposite effect, with polls of voters displaying greater levels of support for Trump than polls that include all adults. I have also included separate effect measures for the two most-common pollsters in this sample, Politico/Morning Consult and YouGov/Economist. Job-approval polls taken by the former organization show a pro-Trump “bias” of about three percent; on the generic ballot their polls place Republican support about five points higher than other polls. YouGov/Economist polls also have Republican tilt on this question, though they show a slight anti-Trump bias in job-approval polls.
If we extrapolate these results to the fall election on November 6th (655 days after the Inauguration), and include the effect for registered voters, the model predicts the Democrats’ lead in generic ballot polls would reach nearly fourteen percent (=4.07+2.62+0.011*655). A margin that large would easily overwhelm the built-in advantage Republicans hold based on partisan self-selection and gerrymandering. Even if the Politico figure is correct, adding in that pro-Republican factor brings Democratic support down to nine points on election day. That result would still reach nine percent, or a Democratic/Republican split of about 54-45. That 54 percent figure still exceeds the 53 percent minimum I estimated earlier would result in Democratic control of the House of Representatives.
Using the model for the relationship between seat and vote divisions presented earlier, a 57 percent margin in the national Congressional vote would translate into the Democrats’ winning 55 percent of the House seats for a margin of 239-196.