This cohort leans Republican now more than ever; a Gallup poll this year shows they favor the GOP by a whopping 25 percentage points. It was as if the Democratic Party didn’t understand how much they needed labor. Oddly, it appears they still don’t. “The working-class voters in my county tend to wonder if the party still wants them,” says Mark Hackel, the Democratic chief executive of Macomb County, Mich., where voters favored Trump over Clinton by 11 percentage points. “They’re not really sure where they fit in.” Mike Mikus, a Democratic strategist who helped guide Pennsylvania Gov. Tom Wolf’s...