... For more than a decade, Rick Perlstein, a historian born in 1969, has pursued the subject of polarization in a sequence of very long books. The latest, The Invisible Bridge, is the third in a project now exceeding 2,300 pages, covering a mere dozen years, 1964 to 1976. The trilogy centers on two defeated insurgent presidential campaigns (Barry Goldwater’s and Ronald Reagan’s), with an insurgent disgraced presidency, Richard Nixon’s, in between.

Along the way, something has happened. A mission that began with every promise of reconstructing the origins of conservative “movement” politics has degenerated into a manic chronicle of what Philip Roth, in a different context, once called “Pure American Dada,” and what Perlstein himself has labeled the “wingnuttism” of the “whackadoodle far-right.” Perlstein’s gift for energetic caricature and his taste for bizarre incidents have overpowered his impulse to sift through the ideas and beliefs that animate his subjects, and to grapple seriously with a politics rooted in authentic if not always coherent dissent....

An insistent vulgarity has overtaken Perlstein’s prose, and it implies contempt not just for Nixon but for the public that eventually elected him president twice, the second time in one of the biggest landslides in history. For Perlstein, the mere fact of a President Nixon is explicable only as pathology. This is the same argument Barack Obama’s unhinged detractors make about him. And just as those detractors depict the pragmatist Obama as the agent of anti-Americanism, so Perlstein describes the centrist Nixon as the sole author of “the fracturing of America,” who feasted on middle-American fears of black militants and campus radicals—even as he turned the federal government into a private militia. After the Kent State protest, in which National Guardsmen killed four demonstrators, vapors of hatred enveloped the victims and, Perlstein writes, “a rumor spread” that one of the four, “whose head was blown off, was such a dirty hippie that they had to keep the ambulance door open on the way to the hospital for the smell.”

Perlstein, who calls himself a “social historian,” now finds rumor more illuminating than fact. His first book drew on more than a dozen archival collections. He has since adopted the methodology of the Web aggregator: his preferred sources are digitally accessed news clippings and TV shows. Some might find this intellectually lazy, but Perlstein proudly Googles in the name of grass-roots activism. “My effort here is about intellectual democracy, in the spirit of the open source software movement,” he explains in the online notes forThe Invisible Bridge, adding, “I welcome messages about errors, omissions, broken links and any questions.” He means it, too: he has included a Gmail address....