This item from NBC's First Read blog is extremely perceptive:
Gut check: Or is it a head or heart check? How many GOP primary voters are finding themselves split on whom to support because their head tells them Romney is the brightest guy to put up against Clinton; their heart wants them to believe Thompson could become the answer; and their gut keeps coming back to the idea of Giuliani? A few other debate thoughts: Did anyone else notice Huckabee disappearing a bit on stage? His normally well-received schtick didn't seem the right pitch for an economic debate, and Thompson definitely seemed to overshadow him. McCain had a couple of good moments, but he also didn't seem to be as involved in this debate as he was in previous ones. By the way, when will the front-running candidates begin banding together and ask debate organizers to shrink the field. All of them would likely benefit.
My own view is that each of the Big Four have strengths and weaknesses in a general election match-up against Hillary Clinton, and that, as you learn in Applebee's America, gut connections often count for much more in politics than rational appeals.