I'm not sure I've ever heard a republican respond to the basic, historical fact about both parties virtually switching roles after the CRA passed. Obviously plenty of southern racists remained in the democrat party, but by 1972 it was pretty clear who was fostering anti-black sentiments and who wasn't. Hell, even in 1968 democrats nominated a relatively progressive Hurbert Humphrey, who was an early advocate for civil rights. The south went to George Wallace, who had left the democrat party due to civil rights issues. And then in 1972 Nixon won all the states Wallace had won four years ago, plus the rest of the south.