President Lyndon Baines Johnson lived 64 years. Robert Caro has been examining that life for about 35.

“I don’t like to be rushed,” he told me early in April. We met in the room where Caro wrote the fourth volume of his Johnson biography. It is not the last. “The Passage of Power” brings Caro’s work on Johnson to more than 3,000 pages. Only near the end of his most recent book has Caro arrived at the beginning of Johnson’s presidency, which lasted from 1963 to 1969.

Each time he releases another volume — usually after an interval of eight to 12 years — Caro provokes astonishment that he would devote so much time to one subject. He’s been compared more than once to Ahab, Melville’s one-legged captain obsessed with Moby Dick.

The comparison is a bit morbid. Ahab’s life did not end well. The Columbia Journalism Review asked a decade ago if Caro’s excessive interest in Johnson had “led him astray,” and his 80-year-old editor has told the New York Times that he doesn’t expect to live to edit Caro’s final book.

But Caro’s meticulous process delivers powerful results. A onetime newspaper reporter, he abandoned that deadline-oriented mind-set long ago. In a world of snap judgments and ephemeral facts, he makes exceptional use of the commodity that modern journalists have the least of: time.

Now 76, Caro exhibits a youthful enthusiasm when discussing his work among the bookshelves and filing cabinets of his Manhattan office. Saying that he would be too “lazy” if he worked at home, he has commuted to this room for decades. Almost nothing in the office betrays the arrival of the 21st century. His inbox is an inbox. He does use a computer for research, as he says the LBJ Library complained about his clacking typewriter.

For his LBJ biographies, Caro supplemented exhaustive digging by interviewing those who knew Johnson, who died in 1973. He temporarily moved with his wife, Ina, to Johnson’s home state of Texas, driving into the countryside outside Austin to sit with the president’s relatives and boyhood classmates. He met other aging sources in their offices, continued calling after they entered nursing homes and preserved his notes for years after they died.

The roster of available interviewees is thinning — “The human life span is my biggest obstacle,” Caro said — but he occasionally finds one more. In 2008, when he learned that Cecil Stoughton, who photographed Johnson’s swearing-in after John F. Kennedy’s assassination in 1963, was still alive, Caro picked up a phone. Stoughton’s wife answered.

“Mrs. Stoughton, my name is Robert Caro,” the author recalled saying. “And she says, ‘Cecil has been waiting on you to call.’ ”

Caro weaves the photographer’s memories into a pivotal section of “The Passage of Power.” In the new volume, Johnson, desperate to lead, was trapped in a powerless vice presidency, irrelevant and despondent. “In the crack of a gunshot it [was] reversed,” Caro said. The new president had to reassure a traumatized nation and begin driving Kennedy’s legislative agenda through Congress.

Having accumulated information for decades, Caro deploys it as ruthlessly as Johnson wielded power. Early volumes of the biography describe a driven young congressman who broke his aides’ spirit and rewarded campaign contributors with federal contracts. Johnson kissed up to anyone who could help him — often literally kissing the bald head of his mentor, House Speaker Sam Rayburn (D-Tex.).

Caro’s prodigious research has not shielded him from criticism. In the 1990s, he was drawn into a debate with the writer Sidney Blumenthal, who accused him of overdramatizing Johnson’s disputed 1948 election to the Senate, idealizing LBJ’s opponent to make Johnson look worse. “What he has overlooked completely is politics,” Blumenthal declared in an epic review in the New Republic, “not to mention a number of facts.”

That’s the other knock on Caro: He’s mean. Years ago, a college professor introduced me to Caro’s work by saying, “He’s a hater.” She said this while discussing Caro’s 1974 Pulitzer Prize-winning book, “The Power Broker,” a 1,246-page investigation of Robert Moses, New York’s indomitable builder of bridges and highways. Still, her opinion did not prevent her from assigning the book.

I found it breathtaking. For all the devastating detail — the great builder wrecked vibrant neighborhoods to make room for new roads soon clogged with traffic — Moses was not destroyed in my eyes. When I discovered Caro’s LBJ series, neither was Johnson. They grew larger, their stories told by a biographer with ambitions as vast as theirs.

Caro writes about flawed, vindictive men who degraded democracy but wielded power brilliantly. Johnson rose from poverty, put young men to work in the Great Depression and got loans to string electric wires in rural Texas, transforming the lives of people as poor as he had been.

In “The Passage of Power,” Johnson extorts favorable coverage from the media and feuds pointlessly with Attorney General Robert Kennedy, but when he becomes chief executive, he shines. Manipulating Senate rules as well as the men who made them, the new president outmaneuvers his fellow Southern Democrats to pass the 1964 Civil Rights Act.

Has Caro grown to like Johnson? “Like” is not the word, he said: “He’s the same guy. In my earlier books, you see a man looking for power and utterly ruthless in his attempts to get it. In my latter books, you see what he does with the power once he gets it, and it is kind of wonderful.”

There’s time for the portrait to darken again, as the fifth book follows Johnson’s administration into Vietnam and social chaos in the wake of the assassinations of RFK and Martin Luther King Jr. Caro has outlined the book on a series of typewritten sheets, tacked across two corkboards in his office. At the end of the outline, he has written the final book’s final sentence, which he asked interviewers not to read.

Unlike Ahab, who never caught his whale, Caro can see the spot where he plans to fling the last harpoon.

Steve Inskeep is a co-host of NPR’s “Morning Edition.” Follow him on Twitter: @NPRInskeep.

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