The actress Kim Novak, a megastar of 1950s cinema, was a near-recluse when friends urged her to take a chance and appear at the Academy Awards last year.

Sitting at home, Donald J. Trump spotted Ms. Novak, then 81, on his television screen and recoiled at her appearance. He tapped out a message on Twitter.

“I’m having a real hard time watching,” Mr. Trump wrote. “Kim should sue her plastic surgeon!”

To Ms. Novak, who read the message after the show, it was a devastating setback in her return to public life: She retreated to Oregon, fell into what she called “a tailspin” and refused to leave her house for days. In an open letter to her fans a few weeks later, Ms. Novak denounced Mr. Trump’s tweet as bullying.

For Mr. Trump, a serial exploiter of old-line media, it was a painful yet potent lesson in the power of new media to agitate and motivate a vast, unseen audience. Inundated with replies of disgust and support, he eventually backtracked online (“I was always a big fan of Kim Novak”) and sent her a rueful private letter.