Sure he can. He's showing himself to be strong in the South, where Romney is weakest. And, for many, his conservative credentials will take some of the bitter taste out of pulling the lever for Mittens. Plus he puts a crease in the race card that the Dems are sure to play. Romney/Perry, which some deluded folks claim is the Rino Master Plan, does none of that.
Unless Cain shoots himself in the foot, he's the VP nominee. Doesn't have the bucks or the organization to head the ticket. He realizes that and will play nicely with Mitt, IMHO.
He doesn’t stir up the same loyalty and admiration that Sarah did in August, 2008.