Jimmy Fallon was on top of the world. Then came Trump.

Once the undisputed juggernaut of the late-night category, Mr. Fallon’s “Tonight Show,” a celebrity-friendly cavalcade of games and gags, has seen its ratings decline in recent months. Meanwhile, his politically pointed competitor Stephen Colbert, who hosts CBS’s “The Late Show,” has closed what was once a formidable gap of nearly one million viewers.

The resurgent interest in left-leaning programming hasn’t helped Mr. Fallon, a former star of “Saturday Night Live” who has built his brand on his all-around entertainer’s skills and down-the-middle tastes. And as Mr. Fallon is well aware, viewers haven’t seen him in quite the same light since an interview he conducted with Mr. Trump in September, which was widely criticized for its fawning, forgiving tone. In a gesture that has come to haunt the host, he concluded the segment by playfully running his fingers through Mr. Trump’s hair.

Mr. Fallon acknowledges now that the Trump interview was a setback, if not quite a mistake, and he has absorbed at least a portion of the anger that was directed at him by critics and online detractors.

“They have a right to be mad,” a chastened Mr. Fallon said in an interview this month. “If I let anyone down, it hurt my feelings that they didn’t like it. I got it.”