Douthat doubts it:
The best reason to think that Palin would have given Romney a much longer and tougher fight than Santorum is that she would have polled in the double-digits from the beginning and might have consolidated the not-Romney vote early, by winning Iowa and South Carolina and clearing figures like Gingrich off the decks. But I still have trouble seeing which of the various crucial states Santorum lost to Romney that Palin would have won. Michigan? Ohio? Illinois?
Bernstein mostly agrees:
Romney excelled at letting the other candidates destroy themselves. And that's something the Sage of Wasilla could be counted on to do.
But she had something that only Cain and Paul had: charisma. And she was the incumbent vice-presidential candidate. The problem, of course, is that she is a deranged fantasist. But in that, she reflects her party more accurately than Mr Plastic.