* "Great Society," civil rights and Vietnam were his legacies
* Caro at work on 5th book in Johnson biography
* Reputation as dogged investigator, no page unturned
By Ian Simpson
WASHINGTON, April 30 (Reuters) - Robert Caro has spent almost 40 years writing his monumental prize-winning biography of President Lyndon Johnson, but says it is not the 1960s leader that held his fascination for so long, but how political power works in America.
With the long-awaited fourth volume of his Johnson biography "The Passage of Power" due out on Tuesday, Caro said he never wanted to write just about the life of the president who rammed through civil rights and welfare laws that transformed the nation but then was destroyed by the Vietnam War.
"What I'm interested in, and what I think all my books are, or what they try to be, is about different aspects of political power," Caro, his voice hoarse from previous interviews, told Reuters by phone from his New York office.
As far as U.S. political power goes, "Lyndon Johnson was the guy who understood that better than anyone else in the second half of the 20th century."
Johnson's "awesome" political skills were such that he even could find a way to break through the partisan gridlock that grips Washington today, the 76-year-old author said.
But how would Johnson do it? Caro could not say.
"It is in the nature of political genius to find a way to solve problems no one else can solve," said Caro, whose voice carries deep inflections of his native New York, with "foind" for "find" and "oar" for "awe."
Caro has written that power not only corrupts, it reveals character. Johnson was a master politician who rose from poverty in the hardscrabble Hill Country of Texas to unrivaled power as president from 1963 to 1969.
As drawn by Caro, he was ruthless, insecure, compassionate, greedy and secretive. As president, Johnson oversaw landmark civil rights and social legislation such as Medicare and Head Start as part of his Great Society program aimed at ending poverty.
'BITCH OF A WAR'
But his presidency ended in rioting and tragedy, destroyed by Johnson's escalation of the war in Vietnam. "'That bitch of a war,' I think was his phrase," Caro said.
"The Passage of Power," published by Alfred A. Knopf, is more than 700 pages long but covers only six years starting in 1958, when Johnson, a Democrat, was Senate majority leader.
The book chronicles his misery as John Kennedy's vice president, his blood feud with Kennedy's brother Robert, and Johnson's seizing the reins of power just hours after President Kennedy's assassination in 1963.
The fifth and final book will cover the rest of Johnson's presidency, including passage of the Voting Rights Act of 1965 that "made it possible for Barack Obama to become the first African American in the White House," Caro said.
"I think I can do this (last book) rather fast, in three or four years. But I don't know why you'd believe me, because every time it takes a lot longer than I think it's going to," Caro said, laughing.
Caro's vast and complex portrait of Johnson is a throwback to 19th century writers who believed a great subject demanded not just a big book, but a shelf of big books.
Caro has been writing about Johnson since 1977, or longer than the Texan was in politics. "The Passage to Power" alone took 10 years to write.
It and the other three volumes in Caro's "The Years of Lyndon Johnson" - "Path to Power," "Means of Ascent" and "Master of the Senate" - total more than 3,000 pages.
"The Power Broker," his Pulitzer Prize-winning 1974 biography of New York public works czar Robert Moses, a dissection of what Caro called the "naked essence of urban power," is another big book at more than 1,100 pages.
Famed for the doggedness he honed as an investigative reporter at New York's Newsday, Caro's quest to get every detail has included sleeping outdoors in a sleeping bag to get a feel for Johnson's beloved Texas Hill Country.
His interviews run into the thousands, and he said he has stumbled on discoveries while poring through many of the 44 million documents at the Lyndon Baines Johnson Library in Austin, Texas.
'TURN EVERY PAGE'
"I had an editor who once told me, 'Turn every page. Don't assume a damn thing,'" Caro said. His sole assistant is his wife, Ina, herself an author.
Caro has won two Pulitzer Prizes in biography, the National Book Award, and the National Book Critics Circle Award twice, among other prizes. Obama awarded him the National Humanities Medal in 2010.
Caro's writing technique is as old-fashioned as the scope of his books is vast.
To give himself a framework, Caro said he spends weeks writing two or three paragraphs setting out a book's theme, then writes the last line. He declined to give the last line of his next Johnson book.
Caro said he types an outline on pages that he tacks in three rows of about 20 feet (six metres) each on corkboard in his office. Then he types an expanded outline stored in loose-leaf notebooks.
He writes three or four drafts in pen on narrow-lined white legal pads. Then he writes more drafts on a Smith Corona Electra 210 typewriter, a model that went out of production decades ago.
He revises constantly, even in the galleys part of production, to give his books the narrative sweep, character and sense of place they need to make them last for generations.
"If a non-fiction writer, a writer of history, wants his writing to endure, the writing has to be at the same level as the writing in a novel that will endure," Caro said.
(Reporting By Ian Simpson; Editing by Jackie Frank)