No, the writes weren’t opposed by Republicans because Southern Democrats hung Republicans. There weren’t many around. Here in Augusta the Democrat Party was called the cracker party. The head of the cracker party in the 1940 through the 1960’s was the fire chief, with muscle provided by the sheriff. If you bad mouthed the party, the fire chief or sheriff a couple of deputies would pull you over and gave you a knuckle memorandum.
MLK jr was a Republican until he died. So we’re the bulk of the civil rights leaders of that time. It’s a serious understatement to say those republicans opposed Jim Crow and black codes. They may not have held office but history shows they were far more effective than any state rep.