I can’t remember where I heard this idea first, but it’s one that has impressed me as having great truthiness. In this post Power Line says:

Bush-haters at John Podesta’s think tank couldn’t resist digging up every negative comment Snow has ever made about the Bush administration. Not only did this help establish Snow’s credibility and integrity, as Goldberg notes, but it also signaled to the conservative base that this is a good and meaningful selection.

They’re referring to comments Snow has made that Bush is, among other things, “something of an embarrassment.”

On the face of it the argument seems to be that Snow isn’t afraid to criticize the President, which establishes his credibility as more than just the yes-man that might be expected to emerge from Fox News. That’s a fair point, but it can only really establish such credibility if the things he said were broadly true. Snow wouldn’t have credibility if, for example, he said that Bush can’t stay awake in defense briefings; that’s a criticism, but it’s demonstrably not true (I hope).

So either what Snow says is true – Bush is a “a classical dime-store Democrat” with a “listless domestic policy” who has “lost control of the federal budget” – and he thereby establishes his credibility, or it’s not, in which case he’s a clueless no-nothing who shouldn’t be trusted to polish the lectern in the press room, let alone speak from it.