Like other prominent liberals in her profession, Ms. Kaplan had long considered defamation suits a way for the wealthy and powerful to try to silence their critics. Last year, Mr. Trump’s campaign sued multiple news organizations for coverage that the president deemed unfavorable or unfair. The technology billionaire Peter Thiel bankrolled Hulk Hogan’s suit against the gossip blog Gawker that ultimately bankrupted the business.

“What’s changed,” Ms. Kaplan said, “and we’ve all seen it happen before our eyes, is the fact that so many people out there, including people in positions of authority, are just willing to say anything, regardless of whether it has any relationship to the truth or not.”

Some First Amendment lawyers say that an axiom — the best antidote to bad speech is more speech — may no longer apply in a media landscape where misinformation can flood public discourse via countless channels, from cable news to the Facebook pages of family and friends.