Lyndon Johnson gets older in this fourth installment of Robert Caro's epic presidential biography and so too -- unfortunately -- does the author. The early chapters are masterful. But Caro's story-telling and analysis begins to sputter in rather worrisome ways as the book builds to its dual climax of the Kennedy assassination and Johnson's rapid command of the presidency. I missed the firm command of his material that Caro showed in his earlier books. The master is 76 years old now, and for all his vigor, he's beginning to miss some steps.

When Caro is at full strength, his narration is defined by three great virtues.

- A great eye for the interplay of noble/base motivations within his subjects. Caro constantly shines a light on WHY events happened, not just what happened. He does this with exquisite sensitivity to the private ambitions and inadequacies that shape public figures' lives. LBJ's turbulent life provides Caro with a great deal to work with, and Caro makes the most of it.

- Relentless reporting. Caro is famous for tracking down ALL the players and witnesses at each juncture. He's just as good at exploring the mass of documents, both public and hidden, that reveal history as it happened, not just as people remember it.

- Generally fine judgment about how to fit all the pieces together. Caro often spends a dozen pages on events that may be only a paragraph in someone else's book, but most of the time, his extra details and insights make the longer ride well worth it.

This new book opens with Caro at his best. The opening chapters do a fine job of showing Johnson, at the height of his powers in the Senate, sabotaging his own career, based on a miscalculation of how the 1960 presidential nomination battle would play out. We learn how Johnson in 1959 believed that the Democratic primaries would decide nothing, letting him stay above the fray and eventually being anointed at the convention as the Democrats' presidential candidate in 1960. We also get glimpses of LBJ's own dread of failing as a presidential candidate. The upshot: Johnson did very little to mount a serious campaign on his own, only to realize with horror that Kennedy was lapping him in the primaries, preparing to win the Democrats' nomination on the first ballot. Belatedly, LBJ jumped into the race, too late to end up with anything more than the vice presidency.

Caro writes very deftly about John F. Kennedy's allure and Johnson's private anguish in taking stock of his younger rival. Then comes a series of blistering scenes where LBJ and Robert Kennedy essentially declare war on each other. Telling this as Caro does is brave stuff. Other historians, more sympathetic to the Kennedy family, have airbrushed out details for a long time or portrayed Johnson as the one to blame for each misstep. Caro shows how dysfunctional everything was on both sides; his account rings true. Such tensions define the fallow years of Johnson's vice presidency, where he is an ambitious man with nothing worthy to do.

Then comes the Kennedy assassination and its aftermath. Here Caro's account is spellbinding but incomplete. He opts for a classic tick-tock narrative, with minute by minute details, all of which are very fascinating -- and all of which portray Johnson sympathetically. But there have been many, many accounts of those four days, and they don't all track the bland assurances of the Warren Commission report. Caro is weirdly passive anytime he gets to any of the famous points of controversy. All he offers in the way of analysis is a one-liner that he found no evidence Johnson was involved in a conspiracy. That's it. I'm no conspiracy theorist, but part of the reason for reading a book like this is to know, in detail, what the non-crazy conspiracy theorists think they've found, and why an expert like Caro believes that each of these supposed clues is irrelevant or bogus.

And then, in the final section, as Caro salutes Johnson's genuinely remarkable skills in getting landmark legislation through Congress, the narrative turns outright flabby. Caro repeatedly flashes back to the same earlier anecdotes for context -- some of them making triple and quadruple appearances within the span of 50 pages. A little bit of linkage is good. But this gets outright embarrassing. He repeatedly quotes his earlier books, each time via the awkward locution "as this author has written." What should have been a summation instead becomes a labored series of codas that don't close: a mechanical rehash that keeps cycling through the same points.

Still, Caro on an off day is better than 95% of biographers. Savor the best parts of the book. Don't be afraid to skim the slow sections. Hope that Caro and his editors can muster one more book to cover the rest of Johnson's presidency.