There’s a way to do this that is less combative and more quiet, reserved, and gentle. When the leftist is going off about the white race this and the black race that, in a disappointed tone, say, “See, I can’t go with you there because I prefer to treat people as people, not as skin colors. That just seems super prejudiced to me.” Their entire ideology is based on treating different races differently, measuring their level of kindness and cruelty against the color of one’s skin, so this cuts all their known arguments off. They might try to shift gears, but they will soon go back to racial prejudice. Then just repeat it: “Yeah, I still prefer to treat people as people and not as skin colors.” Any person publicly bitching about races is gonna start to feel stupid pushing back against that sentence. Also, “racist” is worn out rhetorically so I think “prejudiced” now works better. Also this traps the left because they now openly concede that racial prejudice is acceptable and good, they just don’t lead with that because they know how stupid it sounds to normal people.