You are exactly right. Just a couple days ago I tried to have a reasonable conversation with a lib on a forum, and when he could no longer defend Obama’s lies, he went into a personal attack mode. I didn’t respond after that, preferring to let him think he’d won the issue rather than get into an immature name-calling game with him.
Keep trying to get back to the subject of the "debate." Let them call you names because they're losing, but politely keep trying to bring the debate back to the subject. Suggest discussing the name calling accusations later. What YOU are is not the subject of the conversation.
Before they start calling you names, they'll try to direct the conversation to another subject, such as Bush, or homophobes, or a woman's crotch, just so they don't look foolish when they can't defend Obomas lies. Again, bring them back to the subject of the debate.
Yes, eventually they'll go into name calling mode and stomp out of the room, but when they get there, you've obviously won. They have nothing, and they know it.