Anybody but Romney in the primary.
Personally, it's been my experience when it comes to political disagreement that forcing someone to defend their candidate or position in a public internet debate, using logic, facts, reason, ridicule, or whatever, more often than not it hardens their position. The more they defend their position, the more invested they become in their position.
On the other hand, if someone gets banned for arguing in support of a liberal candidate, it doesn't remove their ability to read posts on FR, and I do believe most of them continue to read. Once they're no longer feeling forced into a defensive position, they're more likely to loosen their attachement to it. It's like the internet equivalent of "take the cotton out of your ears and put it in your mouth." I think it's an effective method.