No serious candidate can go into such an unpredictable arena and expect to answer questions extemporaneously without eventually getting into trouble. On the evidence, both Messrs. Bush and Carson are performing without a net.

Mr. Bush has now made “fixing it” a Plan-B campaign theme, and maybe he will. In Des Moines, Iowa, last Saturday and in Tampa on Monday, Mr. Bush gave a comprehensive speech regarded as powerful and well-received by neutral observers. Iowan Ardys Anderson, who said Mr. Bush was “flat” in an October speech there, said Saturday: “He sounded like he was speaking from the heart . . . I thought he was great.”

Mr. Bush can’t succeed by lowering expectations for his debate performance. If he doesn’t plant in memory the three best parts of that Iowa speech for Tuesday’s debate, he’ll drift sideways and eventually out of the political conversation.

Mr. Carson, like Donald Trump, proves we are in uncharted waters. Mr. Carson is leading in the Iowa polls despite spending two days there in two months. Possibly the semi-opacity of Mr. Carson’s remarks doesn’t matter and charisma will win the nomination. But I doubt it. If he doesn’t sharpen his focus, the substantive and hyper-articulate Marco Rubio and Ted Cruz will erode his support in the primaries. If he had anything resembling their policy chops, he’d be over 50%.