What was the main cause behind the Democratic pickups in House seats? Was it that they were doing something right, either in rhetoric or policy, that the American people liked or was it Republicans doing something wrong, either in rhetoric or policy? Any polls would be appreciated.

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It's difficult to simplify election results to a single "main cause," so here are some of the more prominent ones:

  • Midterm elections are usually bad for the President's party. Wikipedia has a table of midterm results going back to the Taft presidency (1910 midterms), and you'll notice a lot of negative numbers (indicating the president's party lost seats in one or both Houses of Congress). The President's party gained seats or broke even in 1998 and 2002, but those were both held in the shadows of highly unusual situations (the impeachment of President Clinton and the September 11th attacks, respectively), so they are unrepresentative of the broader trend. Given this, we should expect a loss of seats as a baseline, before even looking at factors specific to 2018. In fact, the Republican gain in the Senate is much more surprising, by historical standards, than their loss in the House (there are structural reasons for the Senate outcome, which we may discuss in another question).
  • Midterms are perceived as a referendum on the President, and Trump is historically unpopular. See FiveThirtyEight's ongoing Trump approval ratings, scroll down, select "net approval," and compare Trump against past presidents through Truman. Trump has been consistently underperforming Gerald Ford, who infamously pardoned Richard Nixon. By itself, that's quite impressive, but he's also underperforming nearly every president on that page in net approval, as compared to this point in their respective presidencies. It's impossible to predict whether Trump will become as unpopular as (say) George W. Bush or Harry Truman at the ends of their second terms (click the "8 years" tab to see that), but compared to other presidents who have been in office for the same amount of time as Trump, he's well below average.
  • The Democrats were energized. 2018 had the greatest midterm turnout in a century, at 49.3%. Nate Silver estimates that 60 million people voted for Democrats in 2018. For context, Silver compares this to Republicans getting 45 million in 2010, and Vox compares it to Trump getting 63 million in 2016. In short, a lot of people showed up and voted on election day, and a lot of them voted for Democrats. Both of these numbers together suggest that Democrats had a very effective get-out-the-vote campaign.

Additionally, here are some correlates which may not be causal (or may have a more complicated causal relationship with the outcome):

  • Democratic candidates outspent Republicans by a considerable margin. But people spend more in races they perceive to be "winnable." The Democrats may have spent more because they were gaining in the polls, rather than gaining in the polls because they spent more, or both may be true simultaneously.
  • A lot of Republicans in the House retired in 2018. All else being equal, incumbents tend to outperform new candidates. But at least some of the time, incumbents retire to avoid losing, rather than the party losing because the incumbent retired.

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