It has changed a lot in fact. Weren't the Democrats over a century back the party the southern landowners? It's really since the 50s that you see the current divide developing, deepening, widening. And a lot of that is conscious strategy from politicians, sensing that they can lock in voting groups by taking strong stances on specific issues, that thus become divisive. Just last week I was reading that, back in the 60s (70s?), the GOP was in favour of abortion as part of their support for family planning, and supported gun restrictions. Back then, they thought of these things as general policy issues that require a rational stance; fast-forward to now, and they have been made into identity-defining issues that have to be nurtured. (Which is why stances can seem so out of touch with reality: rational discourse is not the point.) Or at least, that's my understanding of it.