When Gov. Ron DeSantis of Florida convened a round-table discussion about the news media this week, he spared no effort to play the part, perching at a faux anchor’s desk in front of a wall of video screens while firing questions to his guests like a seasoned cable TV host. But the panel’s message was as notable as its slick presentation: Over the course of an hour, Mr. DeSantis and his guests laid out a detailed case for revisiting a landmark Supreme Court decision protecting the press from defamation lawsuits. Mr. DeSantis is the latest figure, and among the most...