History lesson.
The Democrat party of today traces its roots to the Democrat-Republican party, which loosely called itself also the Republican party. Not to be confused with the current Republican party of today. It eventually became the Democrat party, or DNC, of today. And yes, the DNC DID advocate slavery at once point and during the Civil War, most Democrats were aligned with southern states. After the Civil War, the DNC was comprised of two groups; regular Democrats and Dixiecrats. The Dixiecrats stayed true to their racist policies and the rest of the party evolved past racism and would eventually combat it.
The Dixiecrats continued to be part of the DNC until they started breaking off in 1952 and 1956 because they supported Eisenhower for president and identified more with him than the DNC candidates. This led to individual Dixiecrats to leave the DNC for the Republican party. Between 1964 and 1965, more and more white, southern voters leaned towards voting for Republicans as they seemed to identify with the GOP members who DIDN'T vote for the Civil Rights Act. Finally, Nixon seeing an opportunity, embarked on his Southern Strategy and by 1970, the racist holdovers that made up the Dixiecrats finally switched party and became Republicans.
So, yes, there were racists in the DNC...and the Republican party took them in. So, yes, despite origins in combating slavery...the GOP turned into a racist party. Meanwhile, the Democrats were pro-slavery in its origins...became champions against racism.
SIDE NOTE: I wonder how many conservatives who use the argument that the GOP formed to end slavery (which it did), say that a primary cause, or the cause, of the Civil War wasn't slavery? It's kind of hypocritical.