I have no doubt there will be a great deal of examples that people more familiar with american politics then myself can state but one example would be Rebecca Fleton a georgian feminist who is mostly known for being the first female senator (albeit only for one day as a publicity stunt when she filled her late husband's position and then turned it to over to the senator elect).

Her and her husband owned slaves up until the end of the civil war and she remained until the end of her days an unapolegetic racist who spoke out for lynching and against rights for blacks, who she viewed as savage threats to white woman.

However she did in her later life and in her memoirs in particular, turned against slavery as an institution and admitted it was a mistake not because of the treatment of the slaves but because she thought it corrupted the souls of the white slave owners. In particular she viewed the greatest sins of slavery to be children fathered on slaves by slave owners and then condemned to slavery by their own parent and that it was that sin against god which the south were punished for by the civil war.

So she is a case of a slave owner who later rejected slavery while never rejecting the idea of black inferiorty.