Going to extend to 1940 a little.
The dust bowl region, ND,SD, KS, NB and Iowa and Colorado voted for the GOP as a protest vote in 1940 and 1944, because they said the farm crisis didn't end yet.
A more controversial reason was that the tons of German americans voted for the GOP in 1940 because of the GOP's more isolationist stance in 1940.
Although the second was a dead issue once Pearl Harbor happened. Those protest votes probably was a kind of vote that led to a realignment when groups change their habits. Realignment happens when theres a profound shift in demographic voting like switching of support OR major rise in turnout. Just as the 1968 election votes for Wallace, made once staunch democrat voters into hardcore republican voters.
however its evident that most of the decline in FDR's support was due to German americans switching their votes.
If you look at the county level maps there was a switch of lean democrat counties in the midwest+mountain states where there was a lot of ethnic german americans, to lean republican in 1940 and then it hardened in 1944.
Most likely it was soldiers overseas who may not have been able to cast ballots
