WASHINGTON

The debate dominatrix knows how to rattle Obambi.

Mistress Hillary started disciplining her fellow senator last winter, after he began exploring a presidential bid. When he winked at her, took her elbow and tried to say hello on the Senate floor, she did not melt, as many women do. She brushed him off, a move meant to remind him that he was an upstart who should not get in the way of her turn in the Oval Office.

He was so shook up, he called a friend to say: You would not believe what just happened with Hillary.

She has continued to flick the whip in debates. She usually ignores Obama and John Edwards backstage, preferring to chat with the so-called second-tier candidates. And she often looks so unapproachable while they’re setting up on stage that Obama seems hesitant to be the first to say hi.

With so much at stake, she had to do it again in Vegas, this time using her voice, gaze and body language to such punishing effect that Obama looked as if he had been brought to heel. It was a mesmerizing display, and at an event that drew the highest television ratings of any primary debate this year. The momentum Obama had gained from a vivid speech at the Jefferson-Jackson Day dinner in Iowa drained away by the end of the first half-hour.