Urban-suburb counties increasingly look like cities in terms of racial and economic diversity and density. They are less non-Hispanic white than the nation as a whole, and they exceed the national average for residents with bachelor’s degrees. Median household incomes are also high, at roughly $70,000, compared with $60,000 nationally. Like the cities they surround, these communities are heavily Democratic. One example is Montgomery County, Md., near Washington, D.C.

Exurban counties are far more rural and less racially and ethnically diverse, but college graduates are still plentiful, and median household incomes are high, at about $65,000. An example is Douglas County, Colo., outside Denver.

Exurbs lean Republican in their vote choice. Mr. Trump won these counties by about 17 points in 2016. WSJ/NBC News polling this year shows him with a smaller 7-point lead there.

A third type of county, the blue-collar suburbs, tends to have more manufacturing workers than the nation as a whole. They are densely populated but not very racially or ethnically diverse, and bachelor’s degrees are scarcer. Lake County, Ohio, not far from Cleveland, is a good example.