Maybe the whole "ding Rudy over Bernie Kerik" gambit really was unplanned, and just a case of McCain being McCain and running off at the mouth, but I think it's a good play. McCain's mini-revival in the national polls seems to have come primarily from disillusioned Thompson backers, but I think there's a limit to how much support he can peel off the fading Thompson campaign; the movement conservatives who gravitated toward Fred as a non-Rudy alternative are more likely to break for Romney or even Huckabee, I suspect, than for the Senator from Arizona. Ultimately, McCain and Rudy appeal to similar constituencies within the party, and (Ramesh's best efforts notwithstanding) both are regarded with suspicion by similar constituencies. This means that McCain will always be the last choice for an anti-Rudy voter, which in turn means that to win New Hampshire, he'll need to persuade some Giuliani voters to switch over (back over, perhaps) to him. The other candidates can persuade themselves that they can win without going after Giuliani directly, by simply consolidating the anti-Rudy vote and going from there, but McCain doesn't have that luxury. To build himself up to a point where Romney, Thompson and Huckabee supporters feel like they have to rally around him, he needs to tear Rudy down a bit. And the Kerik story is as good a place as any to start trying.
Ross Douthat is a contributing editor at The Atlantic.