Ten months after Inauguration Day, the trend is holding: For late-night hosts, being sharply critical of President Trump is a winning strategy. And that is bad news for “The Tonight Show.”

Stephen Colbert, who has made Mr. Trump a nightly target, assumed the top position in the ratings race in February and has only increased his lead since then. His program, “The Late Show” on CBS, has taken viewers away from Jimmy Fallon, the cheerful host of NBC’s storied franchise, who has lost 21 percent of his audience year over year since the fall season began on Sept. 25. At the same time, Jimmy Kimmel has made ratings gains in the 11:35 p.m. slot on ABC.

Ever since Mr. Colbert leapfrogged Mr. Fallon in total viewers, NBC executives have emphasized that “The Tonight Show” is still the No. 1 choice of viewers in the 18-to-49-year-old group prized by advertisers.