From the antebellum period to the reconstruction era, it was mostly the republicans who were pro-abolition, correct?

Yet, fast forward to the 1940s-60s and it is the Democratic party who is now more favorable to African Americans with an executive order for no segregation in the armed forces and policies in the New Deal that caused many blacks to switch to the Democratic party. It makes sense that Truman was trying to appeal to the black northerner vote for the 1948 election, but why didn't the Republicans do this? They used to be pro-abolition and now what?

Obviously, there is a huge time gap between the periods that I'm comparing, but it still seems strange to me that a party's stance on a matter could just swap like that? Why did this happen?

Thanks!