I have dealt with racist rhetoric from many individuals throughout my life. There are only a few ways to effectively deal with it when you're confronted face-to-face with a semi-anonymous racist. Since many campaign workers seem to be dealing with this issue right now, here's a primer.
First, swallow your outrage. I get upset when I hear racism or sexism. I'm sure you do, too. Just like me, I'm sure a big part of you wants to tell them how ignorant they are. Don't. It won't help. You'll just widen the gulf there.
Second, let them speak. If you hear something along the lines of "I'll never vote for a woman or a black person," ask the person why they feel this way. If they state something along the lines of "you know what 'they' will do if we elect one of 'them,'" ask them what exactly they mean. Listen to what they actually say. Quite often, they'll be unable to answer. Some will be able to formulate some sort of reasonable response.
Third, draw it away from the race divide. Try to draw the responses to the racist rhetoric into areas of policy. What often happens is that if you keep asking "why," the racial issues turn into class issues - and as Democrats, we have a strong upper hand there.
I suggest actually preparing a binder with quotes from all three on specific issues - sort them by issue. Get plenty of quotes, and mark the good ones - even for McCain.
Fifth, do not be a biased advocate, even if you are. If their actual beliefs lead them to match well with McCain, encourage them to vote for McCain. What you're after is trust. What you're after is the idea that race and party truly don't matter. Squeezing someone who authentically believes in McCain's ideals into voting for Obama or Clinton will just foster resentment, even if you find a vote for McCain to be abhorrent. You're far better off being an honest advocate with the person you're talking to than possibly weaseling one more vote for your candidate.
At the end of the day, you likely won't erase the hardened racist views of someone. What you can do, however, is put just the tiniest chink in that worldview. Maybe you'll add to a few other chinks and a bit of rust already in place, and you'll eventually help bring about change in this one person, helping them get beyond race.
Remember at all times, this campaign has the potential to be about way more than partisan politics. Remember Obama's speech? "A More Perfect Union"?
That's what's really important here. Keep it in mind. Let them speak. Guide them to finding real answers beyond race, even if they're not the answers you agree with.