She looked faux-fascinated, as if coaxing him into thinking he was onto something — nodding, head-tilting, performatively squinting, smiling a little, then a little more — a reel of soon-to-be memes, screaming silent bemusement with a hand on her chin.

He looked miserable.

The initial question, at least, should have been fertile terrain for former President Donald J. Trump: a prompt for Vice President Kamala Harris about immigration, a vulnerability for her, and how she might diverge on the subject from her boss, whose policies on the border have often come under withering criticism.

But by the time Mr. Trump got to talking, he had something else on his mind: rally attendance.

Also cats.

It can be said that Ms. Harris was well prepared in leading him astray. After blaming Mr. Trump for helping to tank a congressional border bill, Ms. Harris unboxed an attack line that seemed handcrafted by a team of Trumpologists to enrage him, distracting him with his own vanity.

“I’m going to actually do something really unusual,” she said, addressing the audience at home. “I’m going to invite you to attend one of Donald Trump’s rallies. Because it’s a really interesting thing to watch.”

Smirking, provoking, Ms. Harris ticked through some common Trump digressions, like windmills and the fictional killer Hannibal Lecter. Mr. Trump’s eyes narrowed, and his head cocked to the left.