Since the dawn of the television age, the campaign news conference has mostly been a dreaded and rote affair – the setting for robotic apologies and questions that double as land mines. What good could possibly come of them?

Donald J. Trump has found an answer.

With shrewd showmanship and calculated irreverence, his year-old candidacy has turned the tired ritual of holding court from behind a lectern into a frequently riveting spectacle of self-promotion and media manipulation. His stadium-size rallies for voters are still his calling card. But the intimate news conference is where Mr. Trump has turned political convention on its head.

Let Hillary Clinton deliver polished policy speeches until November. Now that Mr. Trump is the presumptive Republican nominee, he will be busy starring in his news conferences, selling steaks, hotels and, of course, himself.

Fact-challenged claims will be uttered. Punchy slogans will be repeated. Rhetorical walls will be built. And Americans will either be mesmerized or appalled by the performance.