Still, these counties may offer lessons for the 2020 presidential election, illustrating the types of places President Trump would likely need to win again in order to keep the White House—and that Democrats might have to flip to take it back from him.

A Wall Street Journal analysis shows that the counties with “perfect” voting records from 1980-2016 largely have educational attainment levels below the national average, and they all saw slower-than-average population growth between 2010 and 2018. All but one of the counties have median household incomes below the national average, and they tend to be older and much less diverse than the nation as a whole.

From an economic standpoint, they are a mixed bag. Just more than half had unemployment rates above the national average for July, while some were well below that benchmark.

Wisconsin, a battleground Mr. Trump narrowly won in 2016, has four of these counties, the most of any state.