PLAINS, Ga. – The antithesis of Donald Trump and his administration can be studied, at least for those willing to drive 150 miles south from Atlanta, among magnolias, towering pines and seemingly endless fields of cotton, peanuts – and dreams.

Jimmy and Rosalynn Carter live just a few blocks from the center of Plains, population 700, a town that has become a monument to the former president and his uncommon decency.

Carter, who turns 94 next month, has shunned the trappings and riche...