It's hard enough for HN to keep a high level of discourse. A community based around politics will attract partisans and weigh heavily against people's identities. Eventually one group or another will reach enough critical mass to bring the site to one political center or another, and it will be socially impossible to stray too far from that center. Most forums get around this centering problem by explicitly declaring a center. HN does this implicitly by centering around pg's views--a programming social news site founded by Jeff Atwood would have a much different bent.
Likewise, political forums either implicitly or explicitly center around left-wing, right-wing, libertarian, feminist, socialist, or some other default viewpoint where there may be a core community of e.g. leftists, and right-wing commenters who show up to the fight are something of a different social group than the core of people who agree with the site's ideological bent. (Huffington Post comes to mind).
It's a hard and interesting problem though, so if something comes of this discussion I'd be glad to help.
What you basically need is an incredibly thick skin (you will be attacked from both sides of the political spectrum) and remove trolls/bad debaters with an iron fist.