What is most interesting when you look at those county maps is that it shows how the people most affected by FDR’s tyrannical policies (in particular, his ag policies) were the ones that turned against him.
The AAA of 1934/1935 was the point at which a lot of Plains farmers started to see that FDR was a wolf wearing a sheep’s pelt. By 1940, the farmers of the Plains knew FDR had to go.
The south is what kept FDR in power. The Dixiecrats of their day, so to speak. The south was infested with FDR-like thinking - Huey Long was a guy FDR had to compete with for public opinion.
The South would never have voted for a Republican in that era. Whoever won the Democrat nomination was going to win the Southern vote period. In my hometown, the Democrat slate was published on the front page of the paper as a reminder and it wasn't a paid advertisement. The Republican slate, if there was one, never appeared in the paper. The Solid South remained so into the fifties.
Plus the heavly unionized cities of the North.