Why Joe Biden will win rich places but not rich people

In next month’s presidential election, America’s richest and poorest places will likely vote for the same candidate. That fact, peculiar enough on its own, is all the stranger when you consider that rich and poor Americans won’t be voting for the same person. To understand contemporary US politics, you have to understand this apparent contradiction—and the divisions that create it.

In 2016, the counties with the highest and lowest incomes were much more likely to vote Democratic than average. Among the 10% of counties with the lowest household median incomes,...