“The Passage of Power,” the fourth installment of Robert Caro’s brilliant series on Lyndon Johnson, spans roughly five years, beginning shortly before the 1960 presidential contest, including the Bay of Pigs, the Cuban missile crisis and other seminal events of the Kennedy years, and ending a few months after the awful afternoon in Dallas that elevated L.B.J. to the presidency.

Among the most interesting and important episodes Caro chronicles are those involving the new president’s ability to maneuver bills out of legislative committees and onto the floor of the House and Senate for a vote. One of those bills would later become the 1964 Civil Rights Act.

You don’t have to be a policy wonk to marvel at the political skill L.B.J. wielded to resuscitate a bill that seemed doomed to never get a vote on the floor of either chamber. Southern Democrats were masters at bottling up legislation they hated, particularly bills expanding civil rights for black Americans. Their skills at obstruction were so admired that the newly sworn-in Johnson was firmly counseled by an ally against using the political capital he’d inherited as a result of the assassination on such a hopeless cause.

According to Caro, Johnson responded, “Well, what the hell’s the presidency for?”...