“The Incomparable Mr. Buckley,” a new PBS documentary about the singular opinion journalist who animated American conservatism in the decades after World War II, opens with a clip from “Laugh-In.” Dan Rowan and Dick Martin’s goofy comedy show was known for its silly jokes and catchphrases (“Sock it to me!”), but in December 1970 the hosts persuaded William F. Buckley Jr. to come on as a guest and take questions from the cast.

One cast member, Jeremy Lloyd, observing that Buckley always seemed to be seated during his public appearances, asked: “Does this mean you can’t think on your feet?”

Without missing a beat, Buckley deadpanned: “It’s very hard to stand up carrying the weight of what I know.” Everyone burst out laughing. Buckley held his poker face for a moment longer, and then dissolved into a broad grin.

Later in the documentary, which was produced and directed by Barak Goodman and is part of PBS’s “American Masters” series, there is another clip from that “Laugh-In” appearance. Rowan asks Buckley why, after declining numerous invitations, he finally agreed to come on the show. “Because,” he said, “your producer promised to fly me out in an airplane with two right wings.”

Buckley’s wittiness and merry sense of humor weren’t what made him such an influential writer and public intellectual. But without them, he could never have accomplished all he did — especially the galvanizing of young American conservatives into an upbeat movement that could win elections and shape public policy. In 1950, the year before Buckley burst onto the scene with his first book, “God and Man at Yale,” the renowned critic Lionel Trilling declared that liberalism was “the sole intellectual tradition” in America. There were no meaningful conservative ideas, he said — conservatism amounted to nothing more than “irritable mental gestures which seek to resemble ideas.”

Of course that wasn’t true. What Trilling really meant was that American conservatism was dour and uninspiring, devoid of charisma and inward-looking. But all that changed when Buckley came along. He was erudite, brash, articulate — and he brimmed over with charisma. The views he championed, especially in National Review, the magazine of opinion he launched in 1955, had never before been fused into a unified intellectual ethos. Now they were.

As the film shows, Buckley assembled a disparate group of writers and editors, several of whom didn’t even like one another, and set about constructing a conservative pool of thought that drew from different springs. Over time, American conservatives came to see themselves as embracing a coherent set of ideas: Ideas about limited government and individual freedom, about the blessings of the free market, about the genius of the Constitution and its Framers, about the importance of religion and America’s cultural patrimony. Above all, about confronting the threat to freedom posed by the Soviet Union and the forces of international communism.

National Review was only the start.

In 1960, Buckley helped launch Young Americans for Freedom, a conservative/libertarian activist organization that spread to hundreds of college campuses. The liberal writer Sam Tanenhaus, a Buckley biographer interviewed in the documentary, notes that Buckley was liked and admired by so many young people because “he was antiestablishment.”

Maybe for some, but speaking as one of those (formerly) young people, that wasn’t what appealed to me. I joined YAF as a college freshman and began reading National Review a year later. As I wrote many years later, it gave me a thrill to be “encountering words and arguments that gave shape and coherence to my own inchoate political beliefs.” But I was also thrilled by the magazine’s style. There was an irresistible playfulness, a mischievous delight, to the conservatism that Buckley and his magazine embodied. When Time magazine profiled Buckley in a lengthy cover story in 1967, the headline read: “Conservatism Can Be Fun.”

The same combination — ideological clarity, polemical skill, and joy — characterized everything Buckley did. “The Incomparable Mr. Buckley” recounts his support for Barry Goldwater’s 1964 presidential campaign, his own quixotic but influential race for mayor of New York in 1965, his 30-year run as host of TV’s “Firing Line,” his bestselling Blackford Oakes spy novels, and — his towering achievement — the landslide election of Ronald Reagan, who always credited National Review for turning him into a conservative.

The film dwells, understandably, on the most grievous failing of Buckley’s career: his early and strenuous opposition to the civil rights movement. In 1957, National Review published “Why the South Must Prevail,” an odious editorial that argued that as long as whites were “the advanced race,” they were entitled to “prevail, politically and culturally,” over Black citizens.

Unmentioned in the documentary is that Buckley and National Review completely reversed their position on civil rights over the next decade. More than once Buckley acknowledged that he had been wrong. As Tanenhaus has noted (though not in the film), Buckley greatly admired Martin Luther King Jr. and was an early advocate of a national holiday to honor him.

To my mind, the documentary suffers from two lamentable flaws. One is a sin of omission: Nowhere is there any reference to Buckley’s hard line against antisemitism, which he wanted to make radioactive among respectable conservatives. In 1959, he banned from National Review anyone associated with The American Mercury, a magazine that often dabbled in bigotry against Jews. In the early 1990s, Buckley demoted and then fired Joseph Sobran, one of National Review’s most gifted writers and editors, when it became clear that he harbored antisemitic views. Even more dramatically, he publicly broke with Pat Buchanan in December 1991, labeling the prominent paleoconservative an antisemite just as he was revving up a campaign for the Republican presidential nomination.

Sadly, a film that gets so much right about Buckley ends on a note that couldn’t be more wrong.

Buckley died in 2008, seven years before Donald Trump began his hostile takeover of the Republican Party and nearly 13 years before a MAGA mob violently stormed the Capitol. Yet the documentary closes with footage of Trump riling up his supporters and of the January 6 rioters running amok. There is nothing subtle about the implication that Buckleyism spawned Trumpism; one critic says that there were always “nastier” elements in the movement Buckley had quarterbacked and that “by the end it was clear that the nastier forces had won out.”

It is a shocking libel. In every key respect, Buckley had devoted his life to opposing the kind of recklessness and bitterness that Trump and his modern Know-Nothings display. Respect for constitutional norms, for American internationalism, and for reasoned debate went to the essence of Buckley’s conservatism. Trump represents not the final flowering of the Buckley vision but its final defeat. When Trump ran for president in 2016, National Review devoted an entire issue to opposing his White House bid. The cover declared starkly: “Against Trump.”

The movement Buckley nurtured flowered, grew, and triumphed during his lifetime. But it didn’t outlast him. The godfather of the conservative movement was indeed “incomparable.” Invincible, alas, he wasn’t.