When they play the McCarthy/racist/Nazi card (on any issue) as Ann C. points out your passion play with a Church of Global Warming member has reached the point where the argument has been distilled to:
“Well ... you’re dumb! And stupid!” then s/he sits back with a smile and happily chews his/her cud.
I’ve found the best response is to smile back and say, “I do find myself to be a fascinating topic, but let’s try and stick to the issues.”
LOL!