Jonathan Bernstein's Iron Law of Politics.

Great catch by Andrew Sullivan. It comes from Bernstein's "Iron Law of Politics (Beanbag, This Ain't Edition)" post:

Conservative activists believe ... Republican pols are a bunch of wimpy, half-hearted idealists who allow ruthless liberal Democrats, who play this game for keeps, to trample all over them. Indeed, this follows the Iron Law of Politics that everyone believes that the other side is better at the mechanics of politics: the other side is always more ruthless in their exploitation of the rules and willingness to ignore ethical niceties, more tactically adept, better at extracting money from their base, and (depending on who is complaining) either better at ignoring the policy demands of their crazy ideological base in order to win the center or better at addressing the policy demands of the base, while our side uses and then ignores the policy demands of the base.