Donald J. Trump arrived at the second Republican presidential debate dominating the field but facing a critical question: Would he use the moment to move beyond attention-grabbing theatrics and present himself as a candidate many voters could envision as the party’s nominee — not to mention as president of the United States?

But as became clear within minutes, there would be no new Mr. Trump taking the stage at the Ronald Reagan Presidential Library on Wednesday night. Rather than ignore prodding from his opponents — which is what a more traditional candidate who found himself ahead in almost every state and national poll might do — Mr. Trump not only went on the attack, but even focused his attention on candidates like Rand Paul who polls suggest is anything but a threat to him, and George E. Pataki, whose standing is so low he was not even on the same stage but was relegated to a debate earlier in the evening.

More than that, Mr. Trump did not use this debate to move far beyond the familiar and reliably popular lines on issues like deporting illegal immigrants that have become the stock in trade of his rallies. For nearly 30 minutes, not long after the debate began, this most colorful of candidates faded to the sidelines as his rivals debated in detail issues like Syria and how to deal with Vladimir V. Putin, the Russian president, subjects that Mr. Trump appeared to struggle with when the questions came to him.