She turned to him with an arched brow. A quiet sigh. A hand on her chin. A laugh. A pitiful glance. A dismissive shake of her head.

From the opening moments of her first debate against Donald J. Trump, Kamala Harris craftily exploited her opponent’s biggest weakness.

Not his record. Not his divisive policies. Not his history of inflammatory statements.

Instead, she took aim at a far more primal part of him: his ego.

At his rallies, on his sycophantic social media network and surrounded by flatterers at Mar-a-Lago, Mr. Trump is unquestioned, unchallenged and never ever mocked.

That changed over the course of 90 minutes in Philadelphia on Tuesday, when the woman who had never before met him succeeded, bit by bit, in puncturing his comfortable cocoon and triggering his annoyance and anger.

Ms. Harris questioned the size and loyalty of the crowds at his rallies. She said world leaders call him a “disgrace.” And she claimed his fortune was built by his father, recasting a business mogul who proudly boasts of being a self-made man as just another nepotism baby.