“Why is it that each of us holds the beliefs that we do?” Oppenheimer asks, posing his central question in his opening paragraph. “Why do we follow this set of politics, vote for this party, and associate with these people?” For answers, he excavates the lives and writings of six 20th-century intellectuals, politicians and journalists — Whittaker Chambers, James Burnham, Ronald Reagan, Norman Podhoretz, David Horowitz and Christopher Hitchens — who experienced political or ideological conversions, starting left, veering right.

Together, they are a history of the American left during the American century, how it responded to the Great Depression and deep depressions, to world wars and Iraq wars, and some readers may experience the book through that lens. Please don’t be one of them. The individuals are the real story here. Drawing from memoirs, biographies, essays, interviews, and an exhaustive reading of left-wing literature and news accounts, Oppenheimer fashions a chapter for each, showing in every instance that “political identity is always a negotiation, between what it demands and who we are.”

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He starts with Chambers, whose childhood alienation and fantasies of saving the world led him from the left-wing literary ferment of 1920s Columbia University to the Communist Party, to writing for the Daily Worker and New Masses, to going underground and committing espionage for the Soviet Union. What would pry this dedicated man away from his cause, to the point that Chambers would later testify against his close friend, the spy Alger Hiss? “To leave the party wouldn’t simply entail a change of political ideas and loyalties; it would render meaningless all the sacrifices he’s made in communism’s name,” Oppenheimer writes. “And it would, if he proved unable to replace communism with a new and equally substantial belief system, leave him bereft of purpose in the world.”

Chambers’s transformation features many of the forces that turn the book’s other protagonists: shifting realities, new books, mystical moments. Chambers grew disappointed in the party and its imperial standard-bearer; Stalin’s Great Purge horrified him. He embraced books such as Vladi­mir Tchernavin’s “I Speak for the Silent,” which unmasked Soviet corruption and ma­nipu­la­tion. And he discovered prayer, later even recalling a mysterious voice urging him to fight for freedom. “Chambers still felt the calling to fight for the redemption of humanity,” Oppenheimer explains. But the means had changed. “Unable to live without high purpose, addicted to politics, he would fight communism with all his heart and soul.”

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“Exit Right” dwells on these moments of metamorphosis. “It’s during the period of political transition, when the bones of one’s belief system are broken and poking out through the skin, that the contingency and complexity of belief become most visible,” Oppenheimer writes. And belief is what he hopes to understand.

Not all the case studies in “Exit Right” are as compelling as Chambers’s; Oppenheimer certainly leads with his best punch. By contrast, Burnham, the brilliant critic and Marxist philosopher, is dazzling as a thinker but flat as a person — better to read him, I suspect, than read about him — while Podhoretz offers the saddest, smallest tale of all, that of a writer obsessed with his value in “the stock market report on reputations” of New York intellectual life.

For Burnham, a falling-out with Leon Trotsky (their letters to one another are remarkable in their mix of erudition and pettiness) and a realization that the New Deal might really be as far left as America needed to go would nudge him away from the left and the American Workers Party. As one comrade put it, for Burnham the work of the party was “not a vocation but an avocation.” Podhoretz, a gifted magazine editor at the journal Commentary, was devastated after his grating 1967 memoir, “Making It,” received brutal reviews, including by friends on the left such as Norman Mailer. He had yearned for the recognition that comes with writing something transcendent. Instead, he realized that “he wasn’t great,” Oppenheimer writes. “He wasn’t even good. . . . And he was wrecked by it.” In that wreckage, Podhoretz remade his politics, leaving secular left-liberalism for a conservatism steeped in tradition, religion, duty, Israel and America. He also reinterpreted his memoir as a political battle rather than a literary project — and in that battle, those traitors on the left were the enemy. “It was as though he’d been replaced by a version of himself from an adjacent timeline,” Oppenheimer writes.

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It’s comforting to believe that our political convictions emanate from a deep consideration of principles and paradigms, a careful weighing of alternative belief systems, rather than, say, stubborn opposition to our parents, passive acceptance of regional customs or getting mugged. But as Oppenheimer warns, “the grounds of our beliefs are more contingent than we could possibly ever account for.” For left-wing writers such as Horowitz and Hitchens, the impulses to move right sprang from contingencies far more tragic than negative book reviews — the murder of a former colleague for one, the attacks of Sept. 11, 2001, for the other.

Horowitz’s identity was wrapped in the American left: from meeting the Communist Party friends of his parents, to Berkeley and Europe, to the left-wing magazine Ramparts, to his friendship with the Black Panther leader Huey Newton. But it was the murder of his former colleague Betty Van Patter in the 1970s (at the hands of the Panthers, he soon suspected) that led him to question everything about the left and himself. This process unfolded through a series of essays, notably “Left Illusions” in the Nation in 1979 and “Requiem for a Radical” in New West magazine in 1981, as well as Horowitz’s own epiphany moment — almost all these guys point to one —in a Berkeley bookstore, where he realized that his writings on Marxism were lost in the multiplicity of ideas and could easily prove inconsequential or simply false. Suddenly he’s voting for Reagan.

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With Hitchens, “a perpetually disappointed idealist,” Oppenheimer plays shrink to explain how, after 9/11, the Oxford-educated socialist and prolific correspondent became the scourge of the American left and a relentless supporter of the war in Iraq. Ever mocking of lefties turned rightward, Hitchens became one himself, regarding the war as “a chance to see force deployed on the side of the downtrodden after so many years of writing furiously about force being applied against them.” It was also, Oppenheimer argues, a chance for this son of a British naval officer to see the new and old colonial empires atone. “Hitchens, born to Britain and adopted by America, felt doubly responsible, and saw in Iraq the opportunity to redeem both of his nations and their sins.”

I’m a sucker for multi-character intellectual histories, such as Robert Heilbroner’s bracing “The Worldly Philosophers,” on the world’s great economists, or Paul Elie’s “The Life You Save May Be Your Own,” about four American Catholic writers of the 20th century. (Louis Menand’s “The Metaphysical Club” peers from my bookshelf, still unread, quietly judging me.) So I’m entirely predisposed to love “Exit Right,” and I do.

I wish, however, that Oppenheimer had better fleshed out links among his subjects. The tangencies barely graze, or Oppenheimer chooses to share little of them. He recalls Reagan’s obsession with Chambers’s autobiography, “Witness,” details how Hitchens eviscerated Podhoretz as an intellectual “third-rater” and reminds us that a lot of these guys’ lives revolved around the “intellectual eroticism” pulsating at Columbia University. (Aspiring thought leaders, take heed: If you wish to become a fashionable intellectual who suffers a midlife crisis of conviction, attend Columbia.)

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And while the exit from left-wing politics is clear, the embrace of the right is less so. Horowitz’s transformation is straightforward; so is Hitchens’s, if you accept Oppenheimer’s psychoanalysis. But why do Chambers, Burnham or Podhoretz necessarily embrace conservatism after leaving the left? What was in the water?

This brings us back to Reagan, whose presence in this volume initially perplexes. The mainstream liberalism of his younger days was a modest inheritance from his father, one he quickly spent. “Not for Reagan a descent into utopian delusion, followed by a long dark night of the soul, culminating in a baptismal emergence into the light of God, truth, and conservatism,” Oppenheimer admits. “In Reagan’s conversion story there was no conversion at all.” An outlier in this tale, Reagan gradually takes up conservatism and remakes his belief system, informed by Hollywood labor disputes and pro-market promotional work for General Electric during the 1950s and 1960s.

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In Oppenheimer’s telling, Reagan’s path, easier and less self-aware, is mirrored by the country he would lead. “For Reagan, and for tens of millions of Americans who would travel with him to the Right, political transformation wasn’t marked by catharsis and epiphany,” he writes. “It rarely even revealed itself as a transformation. It was life, lived year to year, decade to decade,” with a deceptive sense of permanence “that gave more comfort and was less anxious to bear than a story of discontinuity and change would have been.” After all, few of us have the luxury of working out our political and ideological angst in the pages of glossy magazines and learned journals.

But those are the pages on which Oppenheimer lingers, and his reading is perceptive. “There’s a certain kind of political writing and thinking that can be done only by someone who is in tension,” Oppenheimer concludes in a brilliant and almost unpardonably late postscript. “It’s by putting ourselves — our highest ideals, our most atavistic impulses, our deepest loyalties — in conversation with the world, with vulnerability and conviction, that new possibilities open up. That’s how we discover ourselves as intellectuals and artists. It’s also how social movements are born, how societies evolve, how culture is enriched and our collective imagination is expanded.”

Oppenheimer began with a book about the origins of political beliefs and ended with one about the literary force of political misgivings. They’re both worth reading.