It's a weird (sorry 😆. ENOUGH!) thing about American politics that the parties kind of swapped sides ideologically.
It could happen because each major US party represented different sections of elite interests, instead of class interests. Their support is vertical with their section of the elite allied with certain sections of the population. As demographics cf those populations changed, so party elites adapted.
In Europe, the social democratic parties all came out of labour movements but compromised (or were compromised by) being part of a capitalist democracy. The conservatives were and are parties of the elite getting support from the middle classes protecting relative privilege. These bonds have become strained but that is a different story.
If you look at political values, there is no doubt that Lincoln's republicans were more like today's democrats than the Confederate republicans were. In terms of geography, also.
