Glenn Reynolds (@ Chicago Law School Faculty Blog), talks about tribalism in online political discussion.

So “polarization entrepreneurs” do try to stir up rivalry, and they often succeed. But because — as I mentioned below — politics are more fragmented than polarized, people can often find things to agree on across the Red/Blue divide. In fact, some cynics may suspect that the Red/Blue divide is, at least in part, an effort to prevent people from finding things to agree on that might imperil current political coalitions.