What’s driving this small Republican gain? Individual district factors matter, but so does the fact that it’s a midterm election, which are historically bad for the president’s party. Attitudes toward the president also matter. Both those factors favor Republicans. But a few months ago, I illustrated how congressional approval influences the national House vote when one party controls Congress and the other controls the White House. Voters recognize that both parties have a stake in the government.

President Obama has a 42.4 percent approval rating, and Congress has a 12.4 percent approval rating. Given those numbers, we’d expect Republicans to win the national House vote by 2 percentage points — very close to what the generic ballot shows.

A Republican gain of five to 12 seats is significantly less than one would expect using presidential approval ratings alone as a predictor. If voters cared only about Obama’s performance, we’d expect a much more lopsided result: A Democratic loss of about 25 seats. That seems unlikely. Part of the reason that gains will be kept down is that, at 12.4 percent, this is the lowest congressional approval rating going into a midterm election since the question was first asked in 1974.