Very interesting data. I had no idea that FDR had such a stronghold on the deep south. In 1940 in South Carolina, 95% (!!) of the popular vote went to FDR over Wendell Wilkie. I’m reasonable certain Mr. Wilkie didn’t have to waste any money on TV advertising in SC. How times have changed....
The Civil War and Reconstruction was still very fresh. Heck I’m sure some of those votes in SC were from people that lived through the war. No Republican could have expected to win down there.
I remember seeing a Nast cartoon when I was 9 talking about the “Solid South” and though it was about actual hard ground not bloc voting habits.