They’re all doing pretty good — the average quality of candidate answers is higher than usual. With the exception of Cain, who is not giving any effective responses to criticisms of 9-9-9.
Update: Okay, just as this was getting published Santorum overreached. The problem wasn’t substance, on which Santorum was right, but manners (lack thereof). And he had been doing well up until that point. Romney handled the rudeness well.