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Would have to know more about his character and his economic views, but, granted he checks out, at first glance he'd probably be an Ike figure and very likely a moderate. That would probably be the most sensible thing Republicans could do, at this point. But Petraeus would only have a chance if Republicans were capable of doing the sensible thing, and if Obama's wild popularity utterly disappeared in the next four years (if, say, he did something really stupid--nominating Cabinet members who turn out to have tax problems is a minor misstep, but hardly the kind of stupidity I'm talking about--we're talking Spitzer-level or Bush-league stupidity here).
Frankly, I don't see either of those things happening, let alone both. Seriously.