Technical Appendix for “Not All Midterms are Created Equal”

While poll watchers hang on every little change in numbers like the lead on the generic ballot, this article will ask some broader questions. What factors appear to have influenced that lead since Biden took office? I will address five kinds of factors:

  • the job-approval rating of President Biden;
  • the number of days remaining before the election when the poll is conducted;
  • features of the polls themselves like the types of methods used and the identities of the polling organizations;
  • the influence of key economic variables like inflation and specifically the price of gas;
  • the effect of events like the fall of Kabul and the Russian invasion of Afghanistan.

To evaluate these factors, I am using the complete set of generic-ballot polls archived at FiveThirtyEight beginning after Biden took office and ending on October 28th. I have excluded the sixty so-called “tracking” polls from the sample since they can overwhelm the other polls and impose an unnatural stability to the numbers. That leaves me with 495 initial observations.

To include a measure of Biden’s job-approval, I also downloaded the complete set of Biden approval polls. Each poll in the 538 database is assigned a unique identification number which I used to match the generic-ballot polls with their corresponding approval polls. Not every generic-ballot poll has a matching approval poll; the final dataset consists of 403 polls where both the generic-ballot and job-approval questions were asked.

Gasoline prices are measured weekly while the unemployment rate and the Consumer Price Index are reported monthly. These data come from FRED. I assigned each poll the values of these economic measures depending on the week the poll was conducted. (All polling dates are defined at the midpoint of the interviewing period.) The polling features are reported in the FiveThirtyEight data.

Compiling a list of relevant events is obviously rather more subjective than these other factors. I went through the timelines for the Biden Administration at Wikipedia and chose events which seemed significant or which received broader coverage than most. Here are the ones I identified:

  • the signing of the American Rescue Plan on 3/11/2021;
  • the fall of Kabul, Afghanistan, on 8/15/2021;
  • the bombing of the Kabul airport on 8/26/2021;
  • the signing of the Infrastructure Bill on 11/15/2021;
  • the Russian invasion of Ukraine on 2/24/2022;
  • the confirmation of Supreme Court Justice Ketanji Brown-Jackson on 4/7/2022;
  • the leak of the draft decision in the Dobbs abortion case on 5/2/2022;
  • the release of the decision in Dobbs on 6/24/2022;
  • the passage of the Firearms Bill on 6/25/2022;
  • the testimony of White House aide Cassidy Hutchinson before the 1/6 Committee on 6/8/2022;
  • the FBI raid on Mar-a-Lago in search of classified materials on 8/8/2022; and,
  • the signing of the Inflation Reduction Act on 9/29/2022.

While this list is arbitrary, we will soon see that few events have a direct effect on the generic ballot.