SAC CITY, Iowa—The winner of the presidential race in this rural hub isn’t really in doubt. Sac County, an agricultural community in northwestern Iowa, hasn’t voted for a Democratic presidential nominee since 1988.

Still, the results here, in what is firmly Donald Trump country, will reveal something important about the nation’s political landscape: Whether Democrats can hold on to their shrinking share of the rural vote, or whether Republican dominance in the Farm Belt will grow. (Read about more counties that show how the...