"YOU GUYS," said Lyndon Johnson, "All you guys in the media. All of politics has changed because of you. You've broken all the machines and the ties between us in Congress and the city machines. You've given us a new kind of people . . . Teddy. Tunney. They're your creations, your puppets. No machine could ever create a Teddy Kennedy. Only you guys. They're all yours. Your product." That was the angry yelp of a wounded ex-president who felt the press he had tried so hard to satisfy had driven him from office, the warrior who knew his crusade in Vietnam was finished for sure the day Walter Cronkite joined the doves.

He was wrong and right. Wrong that those guys create politicians or, all on their own, smash political machines. Right that contemporary mass-media politics is a whole new ball game in which the standard calculations we grew up with-about coalitions and platforms and party alignments-fade into irrelevance.

Today the first brick a rational candidate puts in place as he builds his strategy is a media plan: how to get noticed without seeming eccentric, how to set up the dark expectations that will make modest success look like shining victory, how to focus the vision of his skeptical media biographers on the breakthroughs, not the breakdowns, in his checkered history. Then if he wins he has to keep on winning over the guys who by the necessities of their profession have to decide whether his struggles represent courage against tall odds or the flounderings of a flawed personality. As never before, politicians and those who hope to understand them have to tune in to the new wave lenth, lest they miss the signals of success.

This peculiar book adds material for that understanding. It is a massive description of four important media institutions, The Washington Post , CBS, Time Incorporated, and The Los Angeles Times , as they have developed in recent decades up through the Watergate crisis and Nixon's resignation in 1974. Working mainly from his own extensive interviews, Halberstam allays any doubts anyone might have had that these fonts of imagery are manned by passionate human beings with purposes in mind, not by mindless messenger boys, and that their prides and prejudices are forces to be reckoned with at the center of our pational order.

The characters come on thick and fast. The Post is the saga of the Grahams and, lately, of Benjamin Bradlee, portrayed as a sophisticated clone of the oldtime Chicago journalist: "Get the story, beat the opposition, stick it to them before they stick it to us," as Halberstam perceives his philosophy. CBS is Bill Paley riding herd on the Murrows and Moyers who keep straying into profit-risking controversial territory. Henry Luce, the titan of Time , is seen as a savvy Cromwell, ever pressing his moral matrix onto the slippery reality of his times, but also sensing how the tide is turning well before many of his competitors catch on. The Chandlers of The Los Angeles Times produce and advance Richard Nixon and at last turn on him as "a friend to no man." There are many, many others, discovered in dramatic interplay with crises global and personal.

Halberstam's own lucid literary vision shifts back and forth among these enterprises. Along the way he occasionally pokes out a trenchant observation-of "the real power of journalism" as "the power to define," of "the delicate art of spontaneity" in modern television politics, of the waning import of formal officeholding in the age of the celebrity, of war journalism's obsession with "the story ," as in Vietnam, of the ways huge profits inhibit rather than facilitate experiment in journalism.

What is puzzling about this sprawling tour de force is what Halberstam means us to make of it. Only in a section of acknowledgements at the end does he say he sees it as "a book on the rise of modern media and their effect on the way we perceive events," but there is nothing much on the latter except the assumption that the effect is enormous. The four institutions were chosen as "a cross section of the national press," but he can't mean that; these are extraordinary outfits. The Washington Post was chosen "because it has become a serious national newspaper and because this is in part in book about the road to Watergate"-but that would be rather a shorter book. The epilogue stresses the way these new corporate giants seem to be binding themselves into more and more inhibiting organizational tangles, one theme among many in the preceding pages.

The obscurity of Halberstam's thesis throws us back on the interest of the material itself, which is considerable. But there, too, the style of his storytelling in this book is oddly frustrating. It is not just that there are gaps in the long accounts, though it is disconcerting to discover, for example, that such post stalwarts as David Broder, Jules Witcover [no longer with the paper] and Lou Cannon were neither interviewed nor significantly portrayed, and that the Luce story barely mentions Briton Hadden's Time-shaping role or Luce's long and intense relationship with Mary Bancroft, his sprightly Jungian friend and critic. The harder problem is that he so often chooses adjectives over verbs, sacrificing sustained narrative to assertion. Phil Geyelin is "very smart and very sophisticated and very subtle," Frank Stanton is "very good, very intelligent, very protective, and very devious." Halberstam gives up many a chance to round out the substance of his stories, leaving the reader to supply his own memories of how Murrow portrayed McCarthy, what Kennedy told the preachers at Houston, what Nixon said to the press after his defeat in 1962, what Fulbright's famous speech on Vietnam was about. The result is an elusiveness of character-in-action, for all the rich store of anecdote and description.

But it is well worth the read to wander with this sensitive and serious mind behind the front pages, cover stories and television screens. The book is jampacked with food for thought. Even without the recipe, The Powers That Be is nourishing fare for readers hungry to understand the new configurations of power in America likely to dominate our future. CAPTION: Illustration, no caption, by Gary Viskupic for The Washington Post