When President John F. Kennedy died from an assassin's bullet, Lyndon B. Johnson became president of the United States. He finished Kennedy's term, then won his own term in 1964.

As a senator from Texas, Johnson had accumulated perhaps unprecedented power to advance or kill legislation. As vice president, he exercised significant influence only rarely, however, and seemed to become almost invisible to the casual follower of national politics.

His own vice president, Minnesota's Hubert Humphrey, experienced the same outcome after shifting from the Senate to the vice presidency. (This was captured perfectly in 1965 by Tom Lehrer, songwriter/satirist/performer, with his song "Whatever Became of Hubert?")

"The Passage of Power" is the fourth volume of Robert Caro's ambitious multi-volume biography of LBJ, addressing Johnson's rise to the vice presidency but frustrating drop in power.

Caro's earlier volumes have been controversial, massive and masterful. In 1974, Caro changed the craft of biography in the United States with "The Power Broker," a book of more than 1,000 pages about Robert Moses, the powerful New York City urban planner. Despite Moses' relative obscurity outside of New York, the book won awards, sold well for years and altered the way publishers dealt with the craft of biography. Suddenly, long books about less than internationally known subjects became more attractive to publish. Suddenly, obscure journalists (Caro was a relatively youthful newspaper reporter at the time) gained credibility as biographers, wresting the craft away from Ph.D. historians.

LBJ's allies, some professional historians and some jack-of-all-genres reviewers have criticized Caro for painting an unrelievedly dark, sensational picture of Johnson. I have read all four volumes; I have conducted independent research about those charges; I have interviewed Caro. I disagree with the allegation of sensationalism. As for the "unrelievedly dark" charge, I can only wonder if the critics read the same volumes I read.

Despite Caro's success with the Moses biography, nobody -- including Caro himself -- foresaw when he began his research into Johnson's life that the project would consume 35 years and counting, running to at least five volumes (assuming Caro remains in good health). Volume Four covers only Johnson's vice presidency and the early months of his presidency. Caro's next book will deal with the bulk of Johnson's presidency, including the role of Vice President Humphrey.

Oh, yes, Hubert shows up in this new volume, primarily in cameos related to his time as a senator from Minnesota. That other senator from Minnesota, Eugene McCarthy, makes a cameo appearance, too -- years before he decided to run for president, perhaps unseating Johnson. It seems a lock that Humphrey and McCarthy will figure prominently in Volume 5 of Caro's masterpiece.

Steve Weinberg is the author of eight nonfiction books, four of them biographies.