To take an obvious example, I'm sure his friends thought he was a swell guy, but I don't think we need to revisit Hitler's ideas about the Jews. I don't think we need to give much serious thought to theories about moon landing hoaxes or theories about how the Earth is flat either, no matter how nice the people promulgating them may be. We are only given so much time on this Earth; I can think of better ways to spend it than seriously engaging with obvious claptrap.
Everyone I know (and know of) espouses obvious claptrap from time to time. If that is the sieve with which you denote undesirables to remove from your personal sphere, your circle either has a radius of 1 or you delude yourself about your detection abilities.
However at this point "racists" organized and wound up controlling Congress, the Senate, the Presidency and the Supreme Court. The people that you'd like to ignore are currently running the place. Ignoring them might not be the best idea...
It's pretty unlikely that any dyed-in-the-wool Republican is going to hear your rational argument and be so astonished at your doctrine that he changes his mind.
I don't think this kind of exercise is that difficult, and I get plenty of practice on HN talking to people who have very right-wing ideas about the economy. But come on; it's not a way of working miracles. You might persuade your uncle who loves Donald Trump to moderate a couple of positions; you aren't going to turn him away from the Republican Party.
This is the main reason that election campaigns have moved on and focus much more heavily on turning out their own supporters than persuading "undecided" voters, who are a statistical nullity.
Is that what's at issue here? I think very few people -- nearly none -- are actually open to being rationally persuaded to completely flip their political beliefs. People with completely different political commitments usually start from different axioms.
What is a racist? Is this a fixed idea or does it change year to year and depend on the context? Shouldn't we be careful about writing off people based on fluctuating social mores? What about fascists, Marxists, communists, and anarchists? And, say, Trump supporters? Do they deserve basic human respect? Is there any reason not to dehumanize these groups too? Or others who fail our moral tests?