Hey look, billboards in the South calling MLK a communist during the civil rights era. I wonder why that looks familiar? Oh yeah, conservative media today call democrats or any "domestic enemies" communists.
Migration
People tend to stay close to where they are born, meaning their migration patterns tend to be localized to the area or region of their birth. I.e. a person born in Alabama will most likely stay there and if they migrate out it'll be to a nearby state.
This was even truer 100-150 years ago.
In other words, the family that sprung from those Democrats of the 1860s, for the most part, still reside in the South. Who do they vote for? Republicans.
The popular perception of Americans is that they are independent and rootless, but the reality is that people tend not to move very far from their parents.
www.nytimes.com
Hey look, the electoral college map of 1860. Crazy how the democrats are the southern party and the Republicans are the northern party.
It's kind of like the Republicans were the liberal party of that age and Democrats were the Conservatives. I.E. y'all Republicans of today would be Democrats in 1860.
I mean you see Confederate flags flying in Charlottesville, at the Capitol or where any far right militia groups gather. Y'all literally call that flag your southern heritage and are against the tearing down of Confederate statues, statues put up decades after the Civil War in the era of Jim Crow to instill fear into African Americans.
Shoot, y'all are the same people against the teaching of history as it should be taught, the good and bad.