SAN FRANCISCO — Spend too much time with the art of Diego Rivera, and you may find yourself quietly tiptoeing over into the art-for-art’s-sake camp. That happened to me, anyway, while navigating “Diego Rivera’s America,” an engrossing new show at San Francisco’s Museum of Modern Art.

Not cool, right? The doctrine of art for art’s sake is for decadent aesthetes, oozing privilege. Art is supposed to serve some bigger moral purpose. The doctrine was doubtless even more untenable in the 1920s and 1930s, when Rivera — responding to revolutions in Mexico and Russia, to the Great Depression and to the rise of fascism — made his immensely influential murals. A man like Rivera — just look at him — wasn’t about to sit on the sidelines like some wasted flâneur.

But let’s at least try to remember the point of art for art’s sake. It wasn’t to insist (implausibly) that art exists apart from society. It was to say to politicians, collectors, ideologues: Keep your hands off! Let art be an arena for play, for spontaneously arising truth — not for cliches, commerce, propaganda. Art for art’s sake recognized that if you subordinate art to an idea like “the greater good,” you traduce it. You cauterize its deeper human potential, and end up with kitsch.

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As I read it, the doctrine also acknowledged that, in making art that serves a political ideal, serious artists — the ones who aren’t dilettantes — more often find themselves serving naked power. That was the risk run by Diego Rivera (1886-1957), a very serious artist.

The SFMOMA show, which was delayed by the pandemic, is the largest exhibition of Rivera’s work in 20 years. It displays his murals (most of them immovable but brilliantly shown in situ as video footage on giant screens), surrounded by studies and large-scale cartoons, alongside some of his finest easel paintings, from the early 1920s to the mid-1940s.

The show’s capstone is Rivera’s vast mural, more than 70 feet wide and 22 high, that he painted in front of audiences at the 1940 Golden Gate International Exposition. The 10-panel work, known as “Pan American Unity,” was the last of several murals Rivera made in San Francisco. Intended for the City College of San Francisco, it wasn’t installed there until 1961. More than half a century later, it was deinstalled and conserved over a four-year period. Last summer, it was moved to SFMOMA, where it is displayed in a ground-floor gallery. It will be returned to the City College of San Francisco in 2023.

“Diego Rivera’s America” celebrates the artist’s desire, as guest curator James Oles puts it in the catalogue, to “wield his art as an essential weapon — sometimes blunt, sometimes subtle or seductive — in the utopian struggle for greater racial and social equality, security and justice.”

This is rousing, and not out of tune with our times. But it’s also fraught. Art wielded as any kind of weapon — let alone a blunt one — is almost always doomed to long-term irrelevance, no matter how “seductive” it may seem in the moment.

In that century’s first, convulsive half, Rivera had a massive and generally under-acknowledged influence on art up and down the Americas. He went to Europe in 1907 and, as part of Paris’s burgeoning avant-garde, had a front-row seat at the birth of cubism, whose taut and taciturn spatial language he quickly mastered, later adapting it to his own, more loquacious, loose-belted ends.

Until the outbreak of World War I, according to Oles, “aesthetic theories, exhibition reviews and love affairs were all that mattered” to Rivera (as if that weren’t already a lot!). But by 1921, when he was lured back to Mexico by the post-revolutionary government of Álvaro Obregón, everything had changed.

Charged by newfound political convictions and inspired by the Renaissance frescos he had seen in Italy, Rivera began painting murals. He proved himself (as his biographer Bertram Wolfe put it) “one of those monsters of fecundity,” working long days on giant scaffoldings. He and his fellow muralists — most notably José Clemente Orozco and David Alfaro Siqueiros — were attempting to forge a new Mexican identity and to promote new ambitions for Mexico, for the Americas, for the workers of the world.

A trip to the Isthmus of Tehuantepec in southern Oaxaca in 1922 set Rivera on a new course: No longer would he paint compositions loaded with abstruse symbolism. Instead, he would favor everyday Mexican subjects, focusing on labor, family life, festivals and marketplaces. There is much to love about these works showing men and women making tortillas, grinding cornmeal, scavenging, selling flowers and corn cobs, carrying baskets of fruit or flowers, and washing clothes. Their rounded forms are almost complacently generalized, like the sculptures of Aristide Maillol. But Rivera keeps us engaged with odd compositions, amusing expressions and fabulously rich colors.

Still, his most ambitious and influential works were his murals. Between 1923 and 1929, Rivera completed more than 200 fresco panels for government institutions in Mexico. There was something quixotic about the enterprise, which — for all its art world impact — seemed destined to be received with a kind of anticipatory nostalgia. That’s because public, propagandistic art was being superseded — trounced, really — by the movies.

By the end of the 1920s, Hollywood was the fifth-largest industry in the United States, and in the 1930s, the “Big Eight” studios were releasing hundreds of feature-length movies a year. Art that didn’t move — that was tethered, what’s more, to concrete walls — didn’t stand a chance against cinema’s almost indecent seductiveness and potential for mass persuasion.

More immediately, when Obregón’s term as president came to an end, mural commissions in Mexico dried up, forcing Rivera and his colleagues to seek patrons in the United States. Many Americans were already under the spell of Mexico’s much-discussed cultural efflorescence, so Rivera found a receptive audience.

When he came to the United States with his wife, Frida Kahlo, in November 1930, it was not only to fulfill mural commissions and find patrons. It was also to be feted at New York’s newly established Museum of Modern Art, where Rivera was only the second artist to be given a solo exhibition (the first was Henri Matisse).

Was it a problem that Rivera was a known communist? Yes and no. He had made his first trip to the Soviet Union in 1927, staying nine months. By the time he arrived in America, Wall Street had tanked. The country was convulsed by massive labor protests. The Great Depression was deepening (to borrow a line from Philip Larkin) like a coastal shelf. In this context, even powerful Americans were willing to tolerate, if not embrace, the famous Mexican’s politics. But Rivera had to walk a fine line, and the first murals he completed avoided overt political statements, focusing instead on the wonders of America’s dynamic technological, industrial and agricultural sectors.

It’s too complicated to rehearse here the crazed cotillion Rivera subsequently performed with America’s wealthy industrialists (including Edsel Ford and Nelson Rockefeller); the Soviets (who had sent Rivera home from Russia in 1929); the Mexican government (which banned the Mexican Communist Party); the Mexican Communist Party (which kicked him out for being a Trotskyite); American Marxists (who heckled him for working with capitalist imperialists); and art critics (who claimed he was too much of a Marxist to understand America).

All one can ask is: What chance did sincere art stand, entangled in this web of political jeopardy, temptation and compromise? Rivera could do only, perhaps, what he was good at: working, fulfilling commissions, and organizing human types and platitudinous dogma into impressively complex, large-scale compositions.

Don’t get me wrong — they are impressive, both technically and as historic documents. But if Rivera provided a model for what America had until then been missing — a tradition of artist activism — he also provided a cautionary tale. His influence on a generation of American artists, including all those employed painting murals for the Works Progress Administration during the Depression, was enormous. Thomas Hart Benton, Jackson Pollock and Philip Guston were just three among hundreds of American artists who spent periods in thrall to the Mexican muralists.

But most of them eventually recoiled from the influence. The issue wasn’t so much Rivera’s too-towering example or Stalin’s murderousness or America’s increasingly hysterical anti-communism. In the end, it was kitsch. It was Rivera’s need to appeal to “the people,” to find “publicly significant meanings” (Benton’s phrase), which meant statements so generalized they barely applied to anyone.

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In 1932, Rivera published an essay titled “Mickey Mouse and American Art,” in which he extolled the Disney character and his cartoon pals as “the genuine heroes of American art in the first half of the 20th Century.” He predicted that, after the revolution, cartoons — with their standardized drawing, clear messaging and predetermined meanings — would be the people’s most beloved medium. It was a classic Rivera provocation — designed to keep you wondering where his loyalties lay. But it revealed a lot about his own ambitions.

If it means trying to insulate art from society and politics, art for art’s sake is a flawed, not to say ridiculous doctrine. But subordinating art to political imperatives, as Rivera did, is equally absurd and leads directly to kitsch. Instructively, Frida Kahlo set out to do something much more private, idiosyncratic and psychologically charged. She was not only better artistically, but also — as a feminist trailblazer — more potent politically.

Diego Rivera’s America Through Jan. 2 at the San Francisco Museum of Modern Art. sfmoma.org.