Fed up as aides debated whether he should mention civil rights in his first speech to Congress after the assassination of John F. Kennedy, Lyndon Johnson once asked, "What the hell's the presidency for?"

Robert A. Caro asks the same question of history in the fourth volume of his monumental LBJ biography, The Passage of Power (Knopf, 736 pages, $35). If historians cannot give their fullest consideration—eventually five volumes' worth—to a hard-drinking, profane, philandering, corrupt one-term president from the South too often consigned to oblivion for his role in the Vietnam War, then what the hell is history for? 

Passage begins with Johnson's reach for the White House in 1958, and continues through his pivotal role in the election of JFK and his wilderness years as vice president until Kennedy's assassination in 1963. The second half of the book covers the transformative seven weeks between Kennedy's funeral and Johnson's first State of the Union address.

Books like Theodore H. White's The Making of the President 1960 virtually ignore Johnson's indispensability to the Kennedy campaign. Caro hilariously recounts how Johnson, during his whistle-stop tour of the South, would leave his microphone on as he pulled away from small towns like Greer, S.C. "The Yellow Rose of Texas" blared from the train's loudspeakers, and Johnson's mike would pick up his profane asides to assistant Bobby Baker: "Goodbye, Greer. God bless you, Greer. Bobby, turn off that 'Yeller Rose.' God bless you, Greer. Vote Democratic. Bobby, turn off that fuckin' 'Yeller Rose.'"

In the most famous photo of Johnson after the assassination, Judge Sarah T. Hughes swears him in aboard Air Force One as Jackie Kennedy looks on. Caro puts the scene in context: Johnson had asked Bobby Kennedy to nominate Hughes to the federal bench in 1961, but Bobby refused, humiliating Johnson until House Speaker Sam Rayburn intervened and Hughes was appointed. Her presence on Air Force One was a signal.

Just as LBJ was "Master of the Senate," Caro is Master of the Sentence. After Johnson wins his first, crucial vote in the Senate, Caro writes: "Another element had been his decisiveness: his gift, equally rare, not only for sensing in an instant, in the midst of the cut and thrust and parry of debate on the Senate floor, which way the Senate's mood was running on a bill, and not only, if the mood was running in the wrong direction, for sensing the moment at which the tide might be turned, but a gift as well not only for sensing the moment at which the tide might be turned, but for seizing it—for launching, on the instant, maneuvers that turned the tide." Caro laps the sentence back and forth, repeating the words "not only" like waves in that tide. Writing like this is enough to make other historians weep. It is what the hell history is for.

READ: Robert A. Caro's The Passage to Power is on sale in bookstores.