Some of this, I suspect, is because voters in both parties seem to have developed a resistance to falling in line when the news media expects them to. There were numerous reversals of momentum in the 2008 Democratic primaries, some of which seemed to represent an open rebellion by voters against the news media’s expectations of how they might behave. Even candidates as strong as George W. Bush in 2000 — who may have been the best non-incumbent primary candidate ever, with exceptionally strong fund-raising totals and polling numbers — have encountered a few bumps along the road.

Where I think things become potentially dangerous for Mr. Romney is when he plays into this tendency by playing prevent defense, and behaving as though the nomination is his to lose. If Mr. Romney exhibited this tendency during Monday night’s debate, it may have been much worse during Thursday night’s debate in North Charleston.

At one point on Thursday, while delivering a somewhat rambling answer about his tax returns, Mr. Romney explicitly invoked the idea that he wanted to be careful about his disclosures so as to minimize the potential general election fall-out. That’s a perfectly rational strategy for a politician, but it just isn’t good politics to say it out loud in the context of an intraparty debate.