Slavery abolition is a progressive position, and in the Civil War era the Republican party was the more progressive party. In the 20th century there was an ideological shift in the Democratic and Republican parties.

It's pretty clear that the modern Democratic party was the one pushing for civil rights.

"The transition into today's Democratic Party was cemented in 1948, when Harry Truman introduced a pro-civil rights platform and, in response, many Democrats walked out and formed the Dixiecrats. Most rejoined the Democrats over the next decade, but in the 1960s, Lyndon Johnson passed the Civil Rights Act and Voting Rights Act. The civil rights movement had also deepened existing racial tensions in much of the Southern United States, and Republican politicians developed strategies that successfully contributed to the political realignment of many white, conservative voters in the South who had traditionally supported the Democratic Party rather than the Republican Party. These approaches are known as the Southern strategy. Anti-civil rights members left the Democratic Party in droves, and Senator Strom Thurmond, the Dixiecrats' presidential candidate from 1948, joined the Republican Party." [1]

Also, look at a map of the Confederate states. Look at a map today. Why are they mostly red states today? The people/families that lived there then mostly stayed, except for the millions of African Americans that migrated to the north.

[1] https://en.wikipedia.org/wiki/Party_switching_in_the_United_...