Here in 2018, the results of the just-concluded U.S. House elections tracked almost perfectly1 with FiveThirtyEight’s partisan lean metric,2 or how much more Republican- or Democratic-leaning a district is than the country as a whole.

That doesn’t mean that Republicans won all the red districts and Democrats won all the blue districts — in fact, Democrats won or are leading in 40 districts that lean Republican. But it helps us understand the pivotal role the national environment played in deciding the electoral map. Our partisan lean metric is calibrated to a neutral political environment (imagine a world in which the Democratic and Republican presidential candidates tie 50-50 in the popular vote), but this election did not take place in a neutral environment. Indeed, the Democratic-leaning national mood played a crucial role in deciding the electoral map: The swing toward Democrats was almost totally uniform, with a few exceptions (which we’ll get to in a moment).

If you wanted to predict the results of the 2018 midterm elections, you could have done a lot worse than simply adding the generic-ballot average to each district’s partisan lean.