“Signing books helps to raise my name I.D.,” Cain said. “As a matter of fact, it’s not inconsistent to be on a book tour signing books as well as campaigning. Because we have integrated campaign events along with events related to this.”

The campaign has argued that the tour is part of a "50 state strategy" similar to that of President Obama's, aimed at boosting the national profile of the former Godfather Pizza CEO in the way that Obama's Dreams of My Father book did. But unlike Obama, Cain has not built a ground operation in early primary states like Iowa, and his campaign numbers just 30 staff members spread across five states. He has twice as many book signings as campaign events scheduled for this week.

Still, Cain said he won't "apologize" for his book tour, pointing to recent victories in Florida and Minnesota straw polls as validation of his strategy. Cain finished second in the Values Voter straw poll this past weekend, trailing Texas Rep. Ron Paul, and has recently tied for first or second in many national Republican polls.

“I don’t apologize for these events at all and I’m going to do more of them as part of my campaign,” Cain said. "I consider that the whole package in terms of how I’m spending my time. So to pick out one instance or one event and say that’s not contributing, I don’t think so."