He would be a HP if he became a Democrat because extreme nutballs in your party drove him away??
For the record, I've never called the opposite party the enemy.
In addition, I don't see why someone would go from massive FF to massive HP just because their opinion of which party they fit in changed, when their actual views didn't change at all.
When someone is fighting along side you in the trenches, he is an FF. When he decides that he wants to go fight for the opposition and start hurling arrows back at you (as he would presumably be used by the Dems to defeat moderate Republicans), that would make him a huge HP.