I'm not entirely sure that he really would be a weaker candidate than Romney. I used to think so, but I think there's a case to be made that he might actually be stronger than Romney in certain rust belt swing states.
Even if Romney is the stronger candidate on the national level, what matters is not the popular vote, but the electoral college. So it makes no difference if Obama wins Massachussets with 60% of the vote or 65% of the vote, but it does make a difference if he can win states like Ohio and Michigan.
In a recent
USA Today poll, Santorum did better than Romney in the swing states, as well as nationwide. I don't put too much stock in this particular poll, because it's an outlier but I don't think it's necessarily true that Santorum would be weaker than Romney in the general election.