It was an odd and unexpected moment when, on October 18 at the CNN debate in Las Vegas, the normally even-keeled Mitt Romney suddenly lost his cool. Challenged by Rick Perry about once having employed illegal immigrants as lawn workers, Romney initially answered with a chuckle and strained smile; but, when Perry kept interrupting his attempt at a reply, Romney’s temperature shot skyward. “Anderson?” he called to the moderator, and, when no help arrived, he turned on Perry, his voice rising to a shout and his eyes flashing with anger. “Would you please wait?” he barked at Perry. “Are you just going to keep talking, or are you going to let me finish what I have to say?” At one point during the exchange, Romney reached out and condescendingly put his hand on Perry’s shoulder.

Moments later, Romney returned to his usual stiff good cheer. Still, a flush lingered, as did the questions swirling among political commentators. What had just happened? How could Perry have so easily provoked the polished former CEO known for his robotic self-control?

The confrontation hinted that perhaps there was more to Romney’s emotional makeup than the reserved, overly programmed manner that is usually ascribed to him in the press. Indeed, there is considerable evidence that Romney has a genuine temper. “I’m always very surprised when people talk about Romney as stoic, because I never saw that,” says Brian Lees, the former GOP leader in the Massachusetts state Senate. “He got very animated about lots of things, impassioned, and sometimes angry.” Romney himself has acknowledged as much. In a June interview on CNN, he said his sons had come up with a name for any flare-up involving their father: They call it a “Mitt-frontation.”

A certain level of passion is a perfectly normal thing, of course. Yet Romney (whose campaign did not respond to questions for this article) has several times over the years found himself in embarrassing situations, ranging fro...