The "proof" is always that they consider Republicans to be literal card carrying KKK members that if only given the chance would burn the nearest black person at the stake, and someone like that could never have been against slavery, so the parties surely switched at some point since democrats love blacks so much! Of course the premise that Republicans are all racist is completely made up, but they don't consider that, then that would mean anytime they instantly judged a Republican they were actually being rude.