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Note that the response, aside from being shocked, shocked by McLellan's alleged telling of tales out of school, follows the contours of McLellan's actual line: Bush was 'served poorly by his advisors', who are the villains of the piece.
Like you, I suspect McLellan actually does have a conscience, and that what he's penned reflects (in outsider language, no less, a/t Politico) a desire to put it on the record. Some people actually do think like this, even the ones we think are the bad guys.
But to me, this is still largely a pseudo-event. The respondents have had months to craft their responses, since the first drafts started to float around; they've had how many rounds of practice now, being able to assimilate the 'event' of an Bush-insider tell-all and spin it into the broader narrative? And they weren't exactly starting from scratch, either.
They don't even need a plan; even a third-tier crisis PR firm can handle it now, since the basic moves have been 'reduced to practice'. The administration knows how to spin it, the villains know how to absorb the criticism and emerge intact (and thus won't turn on each other), the discipline remains in effect and the center holds, and they change the subject and juke the pseudo-narrative to full effect.
And like I said, they had plenty of advance notice. Public Affairs Press, the publisher, is hardly the maverick confrontational entity it might have been thought of (though I don't think it was) before being gobbled up by another media conglom several years ago. The people who work there are part of the apparatus, willing to leak and prep and shape and craft, because they also want jobs down the road, and to bank favors.