News Analysis: She turned to him with an arched brow. A laugh. A pitiful glance. A dismissive shake of her head. From the opening moments of her first debate against Donald Trump, Kamala Harris exploited her opponent’s biggest weakness: his ego.

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.

Then she stood by and watched, as Mr. Trump did himself a whole lot of damage.

In answer after answer, the former president reminded Americans of his role in so much of what many would rather forget: the deadly and devastating pandemic, his refusal to accept the results of the 2020 election, a bloody siege on the U.S.

Capitol and the fall of Roe v. Wade. He lingered on his criminal charges and praised Viktor Orban, the strongman leader of Hungary.

He defended a false claim that migrants in Ohio are eating their neighbors’ dogs and cats and recycled years-old anti-abortion attack lines that Democrats supported “execution after birth.”