Around noon on Saturday, Nov. 23, 1963, almost exactly 24 hours after the assassination in Dallas, while the president’s casket lay in the East Room of the White House, Arthur Schlesinger, John Kennedy’s kept historian, convened a lunch at Washington’s Occidental restaurant with some other administration liberals. Their purpose was to discuss how to deny the 1964 Democratic presidential nomination to the new incumbent, Lyndon Johnson, and instead run a ticket of Attorney General Robert Kennedy and Sen. Hubert Humphrey.

This example of the malignant malice of some liberals against the president who became 20th-century liberalism’s most consequential adherent is described in Robert Caro’s “The Passage of Power,” the fourth and, he insists, penultimate volume in his “The Years of Lyndon Johnson,” which when completed will rank as America’s most ambitiously conceived, assiduously researched and compulsively readable political biography. The new volume arrives 30 years after the first, and its timing is serendipitous: Are you seeking an antidote to current lamentations about the decline of political civility? Immerse yourself in Caro’s cringe-inducing catalogue of humiliations, gross and petty, inflicted on Johnson by many New Frontiersmen and, with obsessive hatred, by Robert Kennedy.