That surge was widespread, with nearly every age group turning out at substantially higher rates than they had during the 2014 midterms.

Perhaps the most remarkable aspect of the 2018 electorate was how diverse it was. About 73 percent of the electorate was non-Hispanic white, slightly less than the density of that group in the 2016 electorate. In other words, more than 1 in 4 voters in 2018 were nonwhite.

(We’ll come back to that 2008-2014 line.)

That breaks a remarkable pattern. In each of the prior nine federal elections, back to 1998, the electorate has gotten more heavily white in midterm elections and more diverse in presidential ones. The change in 2018 was subtle, but it was real.

Overall, of course, the trend has been toward more diverse electorates as the population of the country has also grown more steadily diverse. That’s why we highlighted 2008 and 2014 on the graph shown earlier: The electorate in the 2014 election was about as heavily nonwhite as the electorate in 2008.

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Which, if you step back for a second, is remarkable. The results in those two elections were very different. In 2008, Democrats received more votes in House races by an 11-percentage-point margin. In 2014, the Republicans had a 6-point advantage.

It’s a reminder that the correlation between who votes and how they vote isn’t as robust as we tend to assume. There have been times, like from 2008 to 2016 (in the box below), where the midterm electorate got more white and Republicans did much better and then the presidential electorate got less white and Democrats gained ground. Outside of that window, though, things often get more messy.

If the relationship between the diversity of the electorate and partisan voting results were direct and robust, Democrats would consistently do better over time as the electorate grew less white. That’s not what we see.

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In 7 of the 16 elections since 1988, the density of the white vote has dropped and Democrats have improved in the House vote, relative to the prior election. That includes six presidential elections.

In 5 of the 16 elections, the density of the white vote increased, as did the Republican share of the vote, including four midterms. In 4 of the 16 elections, the results were mixed.

It’s safe to assume two things, though. The first is that the 2020 election will probably be the most diverse in history. The other is that, as in 2018, that probably isn’t great news for Republicans and for President Trump.