The 2020 electoral map could be the smallest in years. Here’s why.

In a politically divided nation, with attitudes among many voters hardened and resistant to changing, the 2020 general election could be contested on the narrowest electoral terrain in recent memory.

Just four states are likely to determine the outcome in 2020. Each flipped to the Republicans in 2016, but President Trump won each by only a percentage point or less. The four are Pennsylvania, Michigan, Wisconsin and Florida. Many analysts point to Wisconsin as the single state upon which the election could turn.

Shifting demographics, the growing urban-rural divide and...