They come into the House Assassination Committee hearings lugging suitcases of material. They take notes, stare intently at witnesses and charts and photographs and endless reruns of the famous Zapruder film showing President John F. Kennedy's head explode as a bullet strikes.

For years they belonged to a nether world, generally scorned by the serious.But in the skeptical post-Watergate era - with polls showing a majority of Americans doubting the widely disputed Warren Commission findings - they are today at least acknowledged, if not necessarily believed, by the establishment politicians conducting the hearings. Loosely defined as the "critical community" they are vehement disbelievers of the Warren Commission Report finding that Lee Harvey Oswald acted alone.

Within this group, the range of dissent and credibility is enormous.

"We find it extremely uncomfortable being lumped as a group; we like to think we represent the more rational wing," said one member of the Assassination Information Bureau (AIB). About half-a-dozen AIB members work out of a Washington office crammed with assassination files. They propound their theories at salaries of $50 a week from private donations.

But Harold Weisberg, long-time Warren commission critic, and author of several assassination books, says "there is not one expert" among the AIB. Weisberg is purposely absent from the pack of a dozen or so critics at the hearings. The former Senate investigator and Frederick, Md. poultry farmer feels the committee is deliberately "whitewashing" the evidence by skirting crucial questions. He also says of the critics, "There isn't sufficient unity to refer to us as a community. I believe most are irresponsile, well-motivated but unsuited, while a large number are either ego-motivated or crackpots."

Some will tell you with absolute seriousness and conviction that the so-called "umbreall man' 'spotted in the Zapruder film, shot a poison dart into the throat of the president ia his umbrella-weapon. Others list an army of conspirators the size of the Normandy invasion - from J. Edgar Hoover and Richard Nixon and Gerald Ford to rightwing Americans and Cubans and the CIA and the Mafia.

"Forget the name Lee Harvey Oswald. He was just a patsy," said one theorist at the hearings. Some see a conspiracy behind everything and contend with arched eyebrows that newspapers do not print their ideas because of "organized suppression."

The more respectable refuse to dabble in conspiracy speculation but seriously question from available evidence as do many, including John Connally's wife, Nellie, who was the limousine - the theory that a single bullet passed through Kennedy and wounded connally in the jump seat in front of the president. This is crucial to the case because, if it was not a single bullet, one assassin could not have had the time to fire the two shots.

For the next month, these assassination buffs - who like to call themselves "researchers" - and others following the story will be caught in a curious time warp that both compels and repels many, sifting a 15-year-old disaster and the discrepancies surrounding it as if it were yesterday.

For many in the "critical community," pouring over the facts and pseudo-facts has been a full-time, decade-long obsession.

Some have exhausted their own money to publish books of theories unacceptable to most publishers . Others have made a living by giving lectures around the world, writing articles, collecting a small following.

"I like to think I'm being looked at more as a prophet than a crank," says 30-year-old AIB member Robert Katz. He handed out leaflets at the hearings on points the AIB feels the committee will ignore - among other things, what Katz terms "the framing of Lee Harvery Oswald" based on "lack of incriminating evidence."

Katz and others chafed at the committee's choice of Robert Groden, a 32-year-old photo-optical technician, as the sole representer of their world.

Groden testified that films showing Kennedy's head violently wrenching to the back of the limousine "would seem to indicate" that he was shot form the front, not behind - a theory which a panel of medical experts later refuted. Katz agreed with Groden on that point.Katz dismissed, however, a blob at the bottom of one film - which Groden says he believes is a man possibly holding a rifle - as a"Rorschach test for anyone's paranoia. If you stare at anything long enough, you can believe anything."

Some of the more conservative theorists feel Groden's testimonyreflects on them by putting forth easily assailed "far-out" theories. (Not a few people question whether the committee didn't pick him for just that purpose).

A few years ago, Groden became a mini-celebrity with a traveling road show, teaming with comedian Dick Gregory to show his slides and films and appearing on the new defunct "Goodnight America." He was lavishly praised by chief counsel G. Robert Blakey, as playing "no small part in convincing many members of Congress that the Kennedy case should be reopened."

The far end of the spectrum is represented by men like R. B. Cutler, a congenial New England architect in white bucks, seersucker suit and bow tie, who has peppered the committee with his drawings and theories. "How can any one person speak for all of us?" he sniffed. Cornering one member of the committee at the end of a session, Cutler said, "Hi . . . I'm the 'umbrella man' theorist." The staffer said, "Oh yes . . . I've read all your staff."

Cutler then eagerly pursued the man down the corridor, trying to get his day in the sun as well. Cutler's material places "the umbrella man" as one of several assassins. "Of course, he shot the 'flechette' or poison dart from the umbrella as he twirled it," said Culter. "Not too many people believe that theory."

Cutler says, however, that "no one would talk to me until after Watergate. Then some thought, 'Just maybe you're right about our government and conspiracies.'" Seated with him was Richard Sprague, an expert on assassination photographs and once a consultant to the committee. he believes the assassinations of Kennedy, Martin Luther King and Robert Kennedy were all part of a government-inspired plot. Sprague said "my brother wouldn't talk to me for years." Cutler said, "Your brother? My wife wouldn't talk to me."

Like most critics, Sprague said he started to examine the evidence, saw a number of troubling inconsistencies and got hooked. "More than a decade ago, I started cataloging, analyzing and compiling a photo system and I just put my foot in quicksand and have nevever been able to pull it out."

The mysterious "umbrealla man" long has been a subject of discussion. Photographs taken at the time of the assassination show a man opening a black umbrella, although the sun was shining as the motorcade turned in front of the book depository. On the Zapruder film he can be seen moving the umbrella up down rapidly as Kennedy's limousine passes. At the point when the bullet hits the president, and other spectators dive for cover, several photos show the umbrella man calmly sitting on the curb and being joined by another man. Many "researchers" have alleged that he could have been a signal man for an assassination team. Others see the "umbrella man" as a "riduculous" diversion from important evidence.

"That man has been found and has an explanation," said one committee staffer. "He was pumping it up and down to protest Kennedy's father's dealings with pro-Nazi sympathizer Neville Chamberlain." (Britain's prime minister at the outbreak of the war was often caricatured as carrying an umbrella.) As his listener stared, the staff said, "I admit it's subtle," but indicated that the committee believed the explanation.

Blakey has incurred the wrath of some of the press for his gag rule and for what seems a rather grandiose narration at the beginning of each day's hearings. He refused to talk about the hearings until they are over.

Some people have speculated that the hearings will be more of a circus than a catharsis. "How can anyone say that until they've heard all the evidence?" Blakey asks. "This is not a 'Kojak' program where you introduce the characters, create a conflict and resolve everything in an hour with 15 minutes of commercials."

One Capitol Hill policeman assigned to the hearing room displayed his own brand of cynicism. "I think the Warren Commission bungled it and I don't think we're ever going to get the answers. You know why I think they're holding these hearings now? Because, come November, all those voters are going to say, 'Oh yeah, I remember that congressman. I saw him on television.' It's strictly politics."

As the critics follow the ins and outs of the testimony, they talk fluently about bullet trajectories and autopsy reports, how long it takes to load and fire a rifle, whether the bullet found on the stretcher was a plant. They mull over evidence they feel points to an assassin shooting from another window, and ponder the Oswald-Jack Ruby connection.

Some of them gloomily predict the committee - which has already spent more than $2.5 million - will come to the same conclusion as the Warren Commission. "With Gerald Ford [a member of the Warren Commission] scheduled to testify, they're hardly going to zap the Commission," one said. Others feel that public hearings, no matter how inconclusive, are some sort of plus "as opposed to the blindness imposed by the trauma of the time," as Katz puts it.

And for some doubters, the hearings ultimately will be a beginning rather than an end. Weisberg, sitting in front of his TV set, already has a mile-long list of grievances against the committee.

"The right questions are just not being asked They're asking unimportant questions about the rifle scope, ignoring the crucial question: Was the scope mounted at the time of the assassination in a manner that petmitted accurate firing?

The answer is no. When tested, some of the best shots in the world, have been unable to reenact the shots the way Oswald would have had to make them.

"They never even cared to establish just where the bullet on the stretcher came from - and there was available testimony on that in the Warren Commission report. Darrell Tomlinson, a hospital engineer, said he found it after it was jarred from underneath a stretcher mattress and would have been impossible to have fallen there naturally."

David Lifton, a non-believer in the single bullet theory, now appears on WETA's broadcasts of the hearings as a critic. He is troubled by the "vast" divergencies" of testimony so far. In person, he was no less vehement. "If there is an entry wound in the front of the throat, as Dr. Perry has testified, then where's the bullet? It's not on the X-ray. If he's right, the X-ray is a fraud."

Lifton was pelting ideas such as, "Why was the earlier autopsy draft burned?" At the same time he adroitly slipped in that he has a book about to be published.

He is among the critics who seem destined not to disappear after this hearing. They are scrutinizing the material for a new barrage of refutation - thinking ahead to papers and books and lectures and appearances to come, they hope, long after these hearings have concluded.