It’s a charming scene, not much diminished by the fact that Jack Tanner does not exist. He is one of several fictional elements that Scorsese filigrees around the tour-documentary core of “Rolling Thunder Revue”: a character lifted, in a novel crossover move, from Robert Altman’s “Tanner ’88.” That 1988 HBO mini-series followed the long-shot Democratic presidential campaign of the titular character, played by the actor Michael Murphy. His appearance in “Rolling Thunder Revue” is, on its face, puzzling; why, Rob Salkowitz wondered in Forbes, did Scorsese summon a fictional politician to speak to Dylan’s influence “when you probably could have gotten the exact same lines out of a real person like Gary Hart?” But Tanner’s fictionality gives him a peculiar resonance that Hart wouldn’t have had. He’s not a member of a generation; he’s a mirror.

Like “Rolling Thunder Revue,” “Tanner ’88” is a road movie doubling as a generational odyssey. The loose script was written by the “Doonesbury” cartoonist Garry Trudeau, and like the comic strip’s characters, Jack Tanner is a sharp but affectionate satire of a baby boomer in autumn. His early radicalism has aged into a vague, self-satisfied liberalism, warmed by the afterglow of his participation in historical moments, or at least his proximity to them. He is proud to have an F.B.I. file, but there’s nothing very incriminating in it. Watching a Tanner campaign ad stuffed with stock footage from the 1960s, a focus-group member complains: “He had Kennedy, he had Martin Luther King — I mean, where’s his ideas?”

Trudeau, like most of the baby boom’s great bards, makes certain assumptions about where his generation’s center of gravity lies. His archetypal boomer has traded dean’s-office occupations for corner-office employment, but a little sheepishly, walking away from the counterculture without really repudiating it. These boomers have mellowed, yes, but they have also mellowed the country: made it a more liberal, tolerant, open-minded place, a place where a moderate Democratic Leadership Council type like Jack Tanner can propose drug decriminalization or, briefly, make “social justice” his campaign theme. Even the most irreverent versions of this generational self-portrait can never fully evade the smugness they intend to skewer — the implication that their subjects, for all their narcissism, excess and eventual hypocrisy, were ultimately on the winning side of history.

But sometimes history throws a curveball. From the perspective of 2019, the arc of boomer politics does not seem to have bent inexorably toward Tanner’s mushy liberalism; for now, at least, it ends with Donald Trump. According to Pew Research Center polling, boomers now identify with the conservatism of the Republican right more readily than any other political ideology. Today’s politics are shaped far less by the intra-Democratic street fighting of 1968 or Vietnam or Watergate than by the subtler, structural consequences of the Civil Rights and Immigration and Nationality Acts: the black-and-white part of the ’60s, not the Day-Glo coda that dominates the ex-hippie narrative.