Clive Davis links to Norm Geras, who writes:

As the old political saying has it, the Right looks for converts, the Left looks for traitors....

And me three--the "old saying" I've heard was slightly different--well, it's about four years old, as the first time I heard it was in an August 2002 Instapundit post, where Glenn Reynolds wrote:

I disagree with the Christian bloggers on most of their core issues; probably the only thing we're in full agreement on is that the Catholic Church's behavior in covering up its sex scandal has been shameful. We're at odds on cloning, on abortion, and often on birth control and evolution, though the Christian bloggers aren't as unified on those last two issues. But they're always polite.

On the Left, though, we find all these pseudonymous name-calling bloggers whose specialty seems to be abuse aimed at those deviating from the party line. De Long isn't one of those, of course, but this line from his post bespeaks a certain tribalism: "There's still time for Kaus to return to his neoliberal roots."

As the old saying has it, the left looks for heretics and the right looks for converts, and both find what they're looking for. The effect is no doubt subliminal, but people who treat you like crap are, over time, less persuasive than people who don't. If people on the Left are so unhappy about how many former allies are changing their views, perhaps they should examine how those allies are treated.

And certainly, much of the Kossites-versus-the-world/MSM-versus-Kos battles of the past couple of months have been all about exposing and expelling heretics--with both sides claiming ideological purity, even as the general public at large couldn't care less about the players or their issues.

Update: Somewhat related thoughts, here.