Just remind them of the Democrats' murderous history of racism. It's time to wave the bloody shirt again!
And that has relevance today how...?
LOL, seriously. I remember reading in a book one time about a Southern Republican in the 1990s who pretty much made his campaign strategy to court "The New South" (suburban areas and Northern transplants) and get enough Black support, pretty much by pointing out that his Democratic opponent was once a segregationist ... Black voters more or less gave him a, "so what?" response.
Its the most ignorant thing ever. By that same logic, by being American, you are a genocidal racist because we used to own slaves and kill Indians.