As the Republican presidential candidates struggle to finish first or second in the usually crucial South Carolina primary, they seem to be ignoring an important political rule, summed up in the Boy Scouts' motto: "Be prepared."

The presidency is a job that tends to require intense preparation and study. Sometimes presidents have to respond to unanticipated emergencies. But mostly they need to be prepared when they take initiatives or respond to events in a way that is sustainable and credible.

"One can only conclude that Romney's advisers didn't press him on the issue and that he didn't press them to press him." -- Michael Barone

The most visible case of unpreparedness in South Carolina this week was Mitt Romney in Monday night's debate, when he was pressed to disclose his income tax returns -- an issue he should have seen coming.

But Romney hemmed and hawed, visibly uncomfortable, and finally came out with a new commitment to disclose them some time in April. He compounded his problem after the debate by volunteering that his tax rate "is probably closer to 15 percent than anything" and characterizing his income from speeches as "not very much" when, according to disclosures he has made, it was $374,328 between February 2010 and February 2011.

These are answers that would have been shouted down in any genuine practice session. One can only conclude that Romney's advisers didn't press him on the issue and that he didn't press them to press him.

As for Newt Gingrich, when asked on Tuesday what he will say in Thursday night's debate, he smilingly replied, "I have no idea." He said that he prepares by taking advice from his two grandchildren: Smile, be short and clear, and "then I drink a Diet Coke and see what the questions are. It's a little like being a jazz musician. It's all improvisation. I don't worry about it."

But voters may. Improvisation can sometimes produce brilliant music. But in the White House it can produce inconsistencies, backtracking and, as Gingrich's former colleague Susan Molinari put it in a Romney ad released yesterday, "leadership by chaos."

Rick Santorum also smiles when asked whether he does debate prep. He said that his preparation comes from his intense schedule and insistence on answering as many questions as he can.

But that form of preparation can be misleading. In South Carolina he has not been peppered by hostile questions on same-sex marriage as he was in New Hampshire. But it has prompted him to expatiate on the bad effects of the 17th Amendment (direct election of senators) that risks associating him with a fringe cause he does not support.

A candidate and a president cannot be prepared for everything. But these three candidates, vying for the lead or second place in South Carolina, have shown a degree of unpreparedness that cause heartburn among Republican political pros.

Michael Barone is a resident fellow at AEI