We actually tried that in the last political thread to get locked and it basically killed off the thread, because the fact is most of the partisans simply aren't interested in trying to find middle ground. It's easier to just ask a moderator to ban people who disagree with you.
I think what would help enormously is moderation to remove all the posts that don't advance the conversation. Expressing unpopular or controversial opinions isn't the issue. Get rid of the non sequitur exclamations, the personal attacks, the posts that don't actually respond to the quoted text etc and the threads would be much more streamlined and productive. That would probably be a nightmare to moderate but the end result would be worth it.
Fair moderation would be nice but an expressed dictatorship is fine provided we own up to that and not pretend it's even handed. There have been a few blatant examples of moderation to support the moderator's personal politics over the last day or so which rather defeats the point of conversation.
I'd also like to propose the thread abstract. I've seen other forums with this tool and it really helps focus the conversation. The 9/11 thread is an example of poor retroactive moderation when it was decided the "purpose" of the thread was something that was by no means clear by the title or opening post. A thread abstract would have prevented Unauthorized Opinions from being posted in the first place.
At the end of the day I think we need to remember that most people are just regular guys and girls who are not trained to craft logical arguments for debate. They are not used to having to back up their positions in a reasoned and considered way and they are certainly not used to backing up their positions with actual facts rather than emotive response. Let's face it, the vast majority of people holding an opinion on a given issue have never bothered to actually research it. This is after all a country where 10% of the people believe Obama is a Muslim and 45% believe the Earth is less than 10,000 years old.
I know I forget this fairly often, and when I remember that I'm probably dealing with someone who's never really been challenged to provide an intellectual and fact-based support for a political opinion it changes the way I react. We should probably all try to step back as RoboDad suggests and take a breath when conversations get heated.