Since the fall season began, Mr. Fallon’s audience in the 18-to-49 group has plunged by 26 percent, even though he is, at 43, the youngest of the late-night network hosts.
Still, Mr. Fallon does maintain a lead in that demographic, outperforming Mr. Colbert, 53, by 90,000 a night, on average, for the season. He also averages nearly 200,000 more than Mr. Kimmel, the 50-year-old host of “Jimmy Kimmel Live!” on ABC.
All three late-night network shows have shed viewers in the 18-to-49 demographic over the last year, but Mr. Fallon’s program has lost significantly more than the other two.
The recent declines leave the host of “The Tonight Show” in danger of losing his position as the second-place late-night host. Mr. Kimmel’s show has averaged 2.4 million viewers a night, compared with 2.6 million for Mr. Fallon.
A year ago, a battle between Mr. Fallon and Mr. Kimmel was almost unimaginable, given the venerable NBC franchise’s lead of one million total viewers over ABC’s late-night offering.
During the current season, Mr. Kimmel — who, like Mr. Colbert, has made himself into a politically engaged monologuist — has seen his audience grow by 4 percent. Recently, he enjoyed a wave of publicity and warm reviews after he dissected the legislative attempts to repeal the Affordable Care Act. He made the issue personal by letting his audience in on the details of his infant son’s medical procedures for a rare heart defect.
Mr. Fallon has mostly stuck with his fun-and-games persona. While Mr. Colbert follows his monologues with earnest interviews, Mr. Fallon engages guests with game-show-like segments. During a recent episode, he led the actor Denzel Washington and the basketball star Stephen Curry in a game of tossing random objects (a butternut squash, a skateboard) into a basketball hoop. On Monday night, Mr. Fallon went behind the desk to enthusiastically promote the Amazon Echo Show for two minutes.
Mr. Fallon does deploy an impression of Mr. Trump, but it lacks bite. His inability to capitalize on the political moment has been an outlier for the network, which has had late-night ratings successes thanks to caustic sketches centered on the president on “Saturday Night Live,” not to mention Seth Meyers’s lawyerly satirical segments on “Late Night” at 12:35 a.m.
Mindful of Mr. Fallon’s sunny nature, NBC executives had hoped that Mr. Colbert’s surge in the wild early days of the Trump presidency would die down once the national mood had settled. They envisioned a time when this pair of temperamentally different hosts would trade victories week to week. But in the closing days of 2017, audiences have not returned to the network for Mr. Fallon’s wide-eyed style.
Indeed, the November numbers show Mr. Colbert widening the gap, with a lead over Mr. Fallon of 1.1 million viewers.
NBC executives remain hopeful that Mr. Fallon can find a way to hold on to the top position among younger viewers. On the upper floors at 30 Rockefeller Plaza, younger viewers are considered the coin of the television realm, and the median age for Mr. Colbert’s audience is 61, compared with 56 for “The Tonight Show.”
One sign of hope for Mr. Fallon: NBC has the rights to “Thursday Night Football” through mid-December (they belonged to CBS earlier in the season), and Mr. Fallon’s ratings soared on Thanksgiving, when his show aired after the Giants-Redskins game.
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