Kevin Drum states it:
A few months ago I was pretty much rooting for a Perry or a Bachmann to win the GOP nomination because I figured (a) they’d lose big in the general election, and (b) their loss might push the Republican Party back toward the center sooner than otherwise. But it’s just too obvious now that Obama is genuinely vulnerable and someone in the Republican field might well be our next president. That’s why I’m now mostly hoping that the GOP base comes to its senses, holds its collective nose, and just nominates the Mittster. On a day-to-day basis, I suspect Romney would govern about as conservatively as any of the others. But in an emergency, he’s the only one who seems pretty certain of responding in a non-catastrophic way.