On election day in 1960, having campaigned sporadically for John Kennedy in Syracuse, N.Y., Daniel Patrick Moynihan was assigned to serve as a poll-watcher in what his biographer calls "one of the most disreputable districts in the city."

"Just after the polls opened in the morning," Douglas Schoen reports, "Moynihan stormed in, wearing a tweed hat and a trench coat, in search of Republican chicanery. Finding a locked door inside, he began banging on it, screaming that people inside were up to no good. Somebody finally located the keys to the door, opened it, and found the room within empty."

The incident symbolizes a significant part of Moynihan's life, and not only because of its paranoid flavor paranoia is sometimes the only retional attitude available to an American public servant). It shows, above all, Moynihan the outsider, wondering what is happening on the other side of the door. There is also Moynihan the trouble-maker, ready and willing to say what nobody else will say - loudly. Moynihan the perceiver and sometimes the unraveler of mysteries. Moynihan the impetuous, shooting from the hip.

Since he has become a freshman senator, Moynihan's public image has fallen (temporarily, we may be sure) into a period of relatively benign neglect. But in terms of real power, it is potentially the strongest position he has held so far - perhaps, as the very existence of this warmly supportive biography hints, the launching pad for a presidential campaign sometime before the end of this century. This seems a most improbable climax to the Moynihan career, perhaps, but he has been playing against long odds almost since he was born, just before St. Patrick's Day in 1927.

When he issued the controversial "Moynihan Report" in 1965, dealing with the relation between poverty and family structures in the black community, Pat Moynihan's critics failed to notice that this sociological study was in part an out-of-focus self-portrait. He was born of relatively affluent parents, but his father abandoned the family when he was still a child, and he grew up poor in a broken home, shinning shoes, selling Christmas trees, working as a stevedore on the docks of Manhattan to eke out the family income.

En route from New York's tough Hell's Kitchen neighborhood to the Senate, Moynihan followed a curious trajectory, including service at subcabinet level under Kennedy and Johnson, a disastrous fling in New York City politics, a curious sojourn as the Nixon administration's resident liberal intellectual, and vivid periods of service as ambassador to India and later to the U.N. - not to mention a tenured teaching position at Harvard University.

At the moment, he seems to be busy largely taking care of his New York constituents, modestly supporting the causes and positions he favors, and enjoying the rare prospect of being able to hold the same job for six years in a row.

Subjects on which Moynihan's expert opinion has been solicited at one time or another (and greeted with mixed feelings) have ranged from auto safety to the architecture of government buildings. He was responsible for digging out and developing some of the basic ideas that later made Ralph Nader famous. He is known as a champion of Israel, a staunch opponent of Soviet thuggery and expansionism, a scourge of Third World self-righteousness. He must have made some noncontroversial public statements, but one has trouble remembering them.

Given this kind of raw material, it is remarkable that Douglas Schoen manages at times to make Moynihan seem a bit dull. One possible explanation is the prospect that this opus may be slightly revised and updated sometime in the '80s or '90s as a presidential campaign biography - an examination of those who have held the office indicates that, on the whole, the American people relish a bit of dullness in their chief executives. Personality could be as much of a handicap for Moynihan as it was for Harry Truman.

Whatever the reason, Schoen seems to tone down material that a human-interest writer (or a psychobiographer) would welcome hungrily. For example, when he tells of a stepfather who passed briefly through the lives of the Moynihan children, he tosses in a single sentence saying that Pat and his brother Mike "disliked him, teased him, and pulled such pranks as slashing the tires on his automobile."

Sometimes the problem seems to be that material is used because it is available, not because it is interesting or relevant. We are informed, for example, that while Moynihan was in England on a Fulbright fellowship, "He was moved by the death of King George VI in February 1952, and he attended the funeral . . ." That monarch (solemnly enshrined in the book's index) makes no other appearance in these pages.

Schoen also hints at one point that some of the anecdotal material Moynihan supplies on his early life may be more colorful than precise. He does include, nonetheless, one anecdote that might make a person wonder. When Moynihan was ambassador to the U.N. and his leave of absence from Harvard was about to expire, he had to choose between the two jobs. Schoen reports that Moynihan called the dean of arts and sciences and said, "'I've just informed the president I won't be coming back.'

"'I'm so sorry,'" said the dean, and Moynihan promptly corrected him: 'I mean the president of the United States.'" It may have happened that way, of course, but this is a variation on a Harvard joke that is certainly not as old as the university but perhaps almost as old as the American presidency.

Finally, there is the tone of apologia that occasionally intrudes - though, admittedly, some of these statements might sound like objective analysis if the subject were not still active and controversial: "What Moynihan was doing was not mere political opportunism . . ." "Moynihan may be a skilled political operator who is equally comfortable before crowds and in back rooms, yet he holds firmly to a set of identifiable principles." "There may have been some exaggeration in this statement . . ."

Whatever its shortcomings, however, this book has a fascinating subject. Agree with him or disagree (and nobody, including Daniel Moynihan, has ever agreed permanently with all of his opinions), he is a constant and on the whole a very positive stimulus in our society. Schoen does a better job of discussing the ideas he represents than the personality behind the ideas, but that is a real service to readers.