Donald J. Trump’s chief pollster, Tony Fabrizio, had seen just about everything in his three races working for the controversy-stoking former president. But even he seemed to be bracing for bad news.

Mr. Trump had just debated Vice President Kamala Harris, repeatedly taking her bait, wasting time litigating his crowd sizes and spreading baseless rumors about pet-eating immigrants.

Mr. Fabrizio had predicted to colleagues that brutal media coverage of Mr. Trump’s performance in a debate watched by 67 million people would lift Ms. Harris in the polls. He was right about the media coverage but wrong about the rest. His first post-debate poll shocked him: Ms. Harris had gained on some narrow attributes, like likability. But Mr. Trump had lost no ground in the contest.

“I’ve never seen anything like this,” Mr. Fabrizio said on a call with senior campaign leaders, according to two participants.

It was yet more proof — as if more were needed — of Mr. Trump’s durability over nearly a decade in politics and of his ability to defy the normal laws of gravity.

He overcame seemingly fatal political vulnerabilities — four criminal indictments, three expensive lawsuits, conviction on 34 felony counts, endless reckless tangents in his speeches — and transformed at least some of them into distinct advantages.