Herman Cain seemed to enjoy his turn auditioning for the role of Mitt Romney’s chief foil Tuesday night. Luxuriating at the center of the table thanks to surging poll numbers, he basked in the attention of his rivals as every exchange seemed to come back to his signature 9-9-9 tax plan. It’s a good thing Cain relished his moment in the spotlight, because it’s almost certainly downhill from here. The pizza magnate’s ascension has been the product of his well-honed outsider’s shtick, his stirring personal story and his charisma. It’s a package that resonates with Tea Party activists who are...