This level of late-night political content would have been unthinkable when Jay Leno and David Letterman dominated the hour, let alone during the reign of Johnny Carson. Bill Carter, a former TV correspondent for The New York Times and the author of two books on late night, said that Carson was careful never to reveal his real point of view.

“He thought it was important to keep a broad audience, so he aimed for the middle of the country,” Mr. Carter said, adding that even Jon Stewart, the longtime host of “The Daily Show,” didn’t seem to want to be too identified with either party. But now, he said, “comedians have dropped the veil.”

“They’re worried about the country, and they’re going to say they’re worried about the country,” Mr. Carter said.

And what’s behind that veil is solidly left of center. “People have asked why there isn’t a conservative form of late night comedy,” Mr. Carter said. “I think Fox tried it briefly, but there aren’t enough writers to support it.”

Mr. Fallon is perhaps the closest to the old mold. He has always positioned himself as a broadly appealing figure, quicker to engage guests in a round of charades or karaoke than to hurl down any political gauntlet. (Commentators on the left ridiculed him during the 2016 campaign for seeming to go easy on Trump in an interview that involved him ruffling the candidate’s hair.) But even he has perfected a Trump impersonation, and — while he’s still more likely to paint Trump as juvenile, rather than as the existential threat that other hosts tend to see him as — he lampoons the president on a nightly basis.