WHEN a Vesuvius like John McCain tells you that you belch too much smoke and spew too much fire, you know you’ve got a problem.

And Ted Cruz, a Republican freshman in the Senate who has been front and center in his party’s effort to squash Chuck Hagel’s nomination as secretary of defense, has a problem. He’s an ornery, swaggering piece of work. Just six weeks since his arrival on Capitol Hill, he’s already known for his naysaying, his nit-picking and his itch to upbraid lawmakers who are vastly senior to him, who have sacrificed more than he has and who deserve a measure of respect, or at least an iota of courtesy. Courtesy isn’t Cruz’s métier. Grandstanding and browbeating are.

He sits on the Senate Armed Services Committee, and during its final meeting on Tuesday about Hagel’s nomination, he made such nefarious and hectoring insinuations about Hagel’s possible corruption by foreign influences that McCain, who’d gleefully raked Hagel over the coals himself, more or less told Cruz to cool it. It was an unforgettable moment, and one that Republicans shouldn’t soon forget, because Cruz, 42, isn’t simply the latest overeager beaver to start gnawing his way through the halls of Congress. He’s a prime illustration of what plagues the Republican Party and holds it back.

A fascinating illustration, too. On the surface, he should be part of the solution: young, Latino, with a hardscrabble family story including his father’s imprisonment in Cuba and escape to the United States. But Republicans who look to him and see any kind of savior overlook much of what drags the party down, which isn’t merely or even principally the genealogy of their candidates. It’s the intransigent social conservatism, the whiff of meanness and the showy eruptions. It’s what Cruz, who rode a wave of Tea Party ardor to victory in Texas in November, distills.