Reply #5

But it's not a good idea to engage in political discussions with customers (especially those you disagree with) of a busines that you do not own and/or can afford them cancelling their business with you.

Just a word of advice ... These idiots, after you prove them idiots, will turn around and DEMAND that you be fired because YOU were rude/unprofessional to them.

Editted to add:

Sorry, I just read the discussion part and not the part where you recognize that the idiot will play the idiotic victim. My suggestion is to, either, get out in front of it by going to your boss(es) and coping to it and saying that you tried your best to maintain and attempted to change the subject several times, but he just went off and became personally insulting(whether that's true or not, unless the call was recorded, it'll be your word against his and he will most certainly embellish). Then, cry mercy ... saying "I realize I was wrong and will refrain from any political discussions in the future, even when goaded."

Or, you can wait a see whether the idiot calls and makes a stink ... if so, then, apply the above.