But that debate also showed that Mr. McCain’s performances could be uneven. He stumbled over some words. He looked confused at several junctures and was slow on the draw, retrieving his time on occasion to amend earlier answers. At one point, Mr. McCain seemed surprised by a question about whether he believed in evolution. Yes, he said. But when some of his rivals said they did not, Mr. McCain went back. “I believe in evolution,” he said. “But I also believe, when I hike the Grand Canyon and see it at sunset, that the hand of God is there also.”

When the topic strays from foreign policy, Mr. McCain’s interest can fade and he can lapse into his stump speech.

“McCain’s major weakness is looking wooden, and when he’s out of his comfort zone, his sound bites become weaker and his evasions of questions become more obvious,” said David Lanoue, a political scientist at the University of Alabama and an expert in presidential debates.

What lasts from a review of Mr. McCain’s national debates  21 this primary season and more than seven in 2000 that included George W. Bush  is that he relishes direct confrontation. He presents himself as the authority on the broad themes of war and peace, life and death. And depending on his level of contempt for his opponent, he can drip with condescension, even as he sits calmly with his hands folded in front of him, smiling.

“I told you once before, Alan, and I’m sorry I have to tell you again,” Mr. McCain said to Alan Keyes, a Republican presidential candidate, in 2000. “I’ve seen enough killing in my life, a lot more than you have. I know, I know, how valuable and precious human life is.”

Drawing Mr. Romney, a former governor of Massachusetts, into a trap in the CNN/YouTube debate last November, Mr. McCain said, “Well, Governor, I’m astonished that you haven’t found out what waterboarding is.” Mr. Romney said he did know, prompting Mr. McCain to express further astonishment that Mr. Romney would not call it torture.

But in that debate, it was Representative Ron Paul of Texas, an opponent of the war in Iraq, who was the object of Mr. McCain’s most righteous fury. “I want to tell you that that kind of isolationism, sir, is what caused World War II,” Mr. McCain declared, as he doubled back from a question about taxes. After some boos, he continued, “We allowed Hitler to come to power with that kind of attitude of isolationism and appeasement.”