The Democratic party championed white supremacy from the Civil War until the Civil Rights movement of the 60s, very roughly speaking. This post from Martin Hyde explains how things changed from Civil Rights on, pretty well:
Where has the GOP’s electoral base been for the last 40 years? It’s been the Deep South. When did that happen? It happened when Goldwater came out against Civil Rights and Johnson came out for it. Before that, blacks somewhat split between the two parties. FDR had started to draw lots of black voters during the Depression, since many were willing to cross over from their traditional support for the “party of Lincoln”, especially outside the old south, due to his progressive economic policies.
Goldwater made a stand against the Civil Rights Act based on small-government, small-c conservative principles. Many then and to this day accused Goldwater of being a racist–a claim I don’t support, I don’t think his beliefs were insincere, I think he was genuinely opposed to what he saw as an overstep in Federal authority. He supported civil rights, but on a state by state level in a way that didn’t violate the traditional laissez-faire approach from the Federal government in terms of business regulation. Now, I disagree with Goldwater, personally, as I think the approach we took was absolutely necessary.
Since that time we (I say this as a Republican) gained a massive number of racist, hateful white voters. We’ve accepted their presence with a few conceits:
Real Republicans are Republicans for small-c conservative principles, pro-business, pro-growth policies, free trade, a focus on free markets etc. We are not against civil rights.
If racists in the South happen to vote for our economic agenda because they view it as being against the Democratic welfare state, which they view as primarily benefiting blacks, it’s unfortunate that their thinking is informed in this way, but we’ll happily take their votes.
The problem is this has proven to be a devil’s bargain. We didn’t just get their votes, we’ve also had to accept their way of thinking. The party I’ve been a member of my entire life is now primarily represented by what I’m calling the White Nationalist wing, these are the huge number of blue collar Southern whites who dislike blacks, and view the Federal government as a vehicle that taxes them and gives their money to “undeserving blacks” who are lazy welfare leaches.