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 searching for a Romney alternative. “Black walnut isn’t a flavor of the week,” he declared. But the attributes that lend power to his candidacy–including his unwillingness to play by, or even pay attention to, Washington’s rules–will almost certainly derail his chances of winning the GOP nomination.