Cain said former House Speaker Newt Gingrich (Ga.), whom he endorsed as the presidential nominee, could "step into the role" of vice president but expressed doubt that he could be Romney's running mate.

"I believe that is 50 percent of the decision. The other 50 percent: Who is going to bring some excitement to the ticket," Cain said. "With all due respect, I don't think Newt would bring that excitement, because he has been one of the candidates and because of where he is right now."