Bigger-than-life portrait of LBJ; Biographer Caro traces early career in first of three volumes; The Years of Lyndon Johnson -- The Path to Power, by Robert A. Caro. New York: Alfred A. Knopf. 829 pp. $19.95.

This biography, the first of three projected volumes on the 36th President, is a useful and important book because it shows the difficulties of making our democratic system work in this divided and turmoil-laden world.

It is valuable because it illustrates in practice how one President made it work efficiently, at least as far as his own goals were concerned.

You will find it fascinating to read. You will find it revealing to ponder.

It comes at a good time because presidential-congressional confrontation and stalemate will likely be with us for some time.

Robert A. Caro, a 1975 Pulitzer Prize-winner for his biography of Robert Moses, is eminently qualified. He has researched LBJ, the man and the President, exhaustively, as the book's extensive notes and index attest. He proves himself honest and objective. He deserves to be read. He makes politics come to life.

Here is Caro describing how Lyndon cozied up to FDR:

''During the 1940 campaign, Roosevelt and Johnson had had several discussions. Now (after the election) they were to have more.

The President and the young man talked together in the Oval Office - one can imagine Lyndon Johnson's feeling during those conversations in that bright, sunny room in which he had for so many years longed to sit. And they talked together in the upstairs, private, quarters of the White House. Johnson had breakfast in Roosevelt's Spartan bedroom, the President sitting up in bed with a blue Navy cape around his shoulders. He had lunch in the President's private study, a comfortable room with chintz-covered furniture and walls so full of naval paintings that they seemed almost papered with them, dining with Franklin D. Roosevelt off a bridge table.''

This is more than a biography of a remarkable character from Texas, a schoolteacher and small-town politician who became President of the United States. It is the story of the early career - from childhood to congressional success and then defeat in a bid for the Senate - of a man who learned how to accomplish things in Washington and accomplished much that was worthy and beneficial to the country.

He was restless and frustrated when nothing was going on, and he was ecstatic when he could put his hands on the levers of government and make things happen.

LBJ becomes a bigger-than-life personality under the vivid style of Mr. Caro.

He was not a narrow partisan. He was far more of a pragmatist than an ideologue.

Caro cites Johnson as the most effective money-raiser in American political history.

In flash-forwards, the author points out that Lyndon sometimes showed himself jealous of the graceful and witty John F. Kennedy, and he felt that he should have been the Democratic presidential nominee instead of Kennedy in 1960 and that he would have won decisively instead of narrowly over Richard Nixon.

LBJ's foremost achievement, future historians will no doubt rule, was the enactment of the 1965 Voting Rights Act, which guaranteed that every eligible voter, regardless of race, creed, or color, would be protected in the exercise of that right.

He worked a near-miracle on Capitol Hill, coaxing, cajoling, and pressuring Congress to put this into law. Congress responded. This was LBJ at his best.

In this volume, of course, Caro doesn't deal with the way Johnson would misuse the Gulf of Tonkin incident to extract an unwise, open-ended authorization for the President to do anything he wanted in Vietnam. But in taking the measure of the man, one must consider that act alongside another on which Caro will undoubtedly focus later - an act of which the nation and Johnson himself could be proud. He put all his energy and his ingenuity into trying to bring an honorable end to the fighting in Vietnam. To remove any taint of partisanship from his efforts, he removed himself from elective politics and announced that he would not seek another term as president.

This was a selfless act. It was LBJ at his best.

Mr. Caro can also write with directness and candor in assessing the darker side of LBJ's character, apparent even in the early years of his career. Here is a good example:

''A hallmark of Johnson's career has been a lack of any consistent ideology or principle; in fact of any moral foundation whatsoever - a willingness to march with any ally who would help his personal advancement.''

I doubt if many readers will want to take this lengthy, fact-filled book in a few gulps. It would be better to sip it a little at a time, savoring its taste. It would be worthwhile to do so.

And if, by chance, the readers of this biography should feel that it is too friendly, too sympathetic, they have at hand a quick and potent antidote.

I refer to ''Lyndon B. Johnson: A Memoir,'' by George Reedy (Andrews & McMeel , 4400 Johnson Drive, Fairway, Kan. 66205, 159 pp., $12.95).

Ted Findley, promotion director for the publisher, warns prospective buyers of what to expect. He says:

''This is a warts-and-all portrait of Johnson by a close associate who later broke with Johnson, so the book carries many firsthand observations.''

If Mr. Reedy was tempted at any time to mute his report of the scars of character that caught his all-seeing eye, he successfully resisted that temptation.

So that readers may know what to expect, here is a sample:

On Johnson the man: ''There were some terrible moments - drunken, aimless wanderings through a hotel corridor in Chicago (fortunately blocked off by police) in which he tried to crawl into bed with a female correspondent.''

On Johnson's view of women: ''His view of the female role in society was not very flattering. They justified their existence as bedmates, cooks, housekeepers , mothers, and secretaries.''

This memoir - an antidote, yes; but it strikes me that it concentrates more on warts than on worth.