On 'Liberal': Michael Totten takes on the excommunicating wing of the Democratic Party:

Ideological lockdown is a symptom of a movement in decline . . . politics isn't a game of football, nor is it war. It is okay if you think the other side is right once in a while (most people do, after all) . . . I can’t help but think the intended audience for public heretic-banishing isn’t the target him or herself. It’s the heretic-banisher’s comrades. People on the losing side of political arguments know their support is bleeding away, so dissidents are furiously denounced as an object lesson for anyone else who might waver. It’s a form of damage control, which is why they don’t care if the tactic doesn’t make them any new friends.

To switch teams a bit, this reminds me of what John Derbyshire once wrote about his days as a Marxist. I'm working from memory: he said that the Marxist clique would quickly split into the "pure" and the "heretics" over some obscure point of dogma. The heretics would be cast out, form their own group, and begin the purifying process anew. So it goes.