As most people familiar with US politics will be aware there used to be a sizeable contingent of conservative Democrats and liberal Republicans, not just in the sense that the phrases are used now to indicate that they were conservative or liberal relative to their parties as a whole but liberal or conservative in an absolute sense, so it was perfectly possible for Democrats to be way more conservative than conservative Republicans like Ronald Reagan.
I can understand the division among liberals between the two parties a little better because even when they aligned on social issues, liberal Republicans tended to be more conservative about things like spending and unions. The conservative split is harder to explain.
It was obviously partly a race and regional thing thing, but presumably there must have been reasons for conservative Democrats and Republicans to be in different parties in the first place, so what were the underlying differences between the two groups and why have they ceased to be relevant? The only obvious policy difference I can see is that Southern conservative Democrats tended to favour a more active foreign policy whereas the Mid Western GOP conservatives like Senator Robert Taft tended towards a more isolationist position.