But when you actually look at individual-level voter data, you find something different: Only 12 percent of Democratic primary voters are young and white and moderate. That’s far fewer voters to go around, especially when you’re also competing with, say, Pete Buttigieg for the same voters.

What gives? Well, these various characteristics are correlated with one another, so you can’t just multiply the different numbers together to come up with the right number of voters, which would imply that they were independent from one another. And they’re correlated in ways that are not helpful for Beto (or Buttigieg). Younger Democrats tend to be more liberal than older ones. And white voters — not all whites, but the ones who vote in Democratic primaries — are more liberal than minorities. There are some young, white, moderate Democrats, but not as many as you’d expect.

Conversely, there are more young, white, liberal voters (i.e. the Bernie Sanders base) than you’d expect in the electorate than if these characteristics were uncorrelated, and also more old, white, moderate voters than you’d expect (i.e. Joe Biden base). These are the “hot spots” in the electorate, so to speak.