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Quit arguing with people who say they will vote for McCain if Hillary doesn't get the nod.
They are either Republicans trolling to make us scratch each other's eyes out, or they TRULY believe and want to do what they are saying they will do. They are willing (for whatever reason) to vote against their own interests. Its like trying to convince a suicide bomber who is strapped up and ready to detonate that his religion is false and that he doesn't have to detonate himself. It cannot be done. They are invested and that's that.
So let them go. Encourage them to vote fro McCain. Wish them well. You cannot and will not change their minds. It doesn't matter anyway.