"King," NBC's six-hour, three-part film on Dr. Martin Luther King Jr. and the civil rights movement, starts out as a cumbersome and heavy-handed situation tragedy, but eventually evolves, in about its fifth hour (to air Tuesday night), into a stirring and absorbing portrait of a man and a mission perfectly matched.

Even at its conclusion, however, "King," which begin with a two-hour telecast Sunday at 9 p.m. on Channel 4, doesn't inspire the tremendous emotional release one hopes for and expects. This is probably because writer Abby Mann mounts a high horse of sanctimonious preciousness at the start and insists on parading his own credentials as a proudly guilty liberal.

It doesn't make King any more heroic a figure to portray John and Robert Kennedy as spineless vacillators on the civil rights issue; Bobby is depicted as something of a quivering neurotic who doesn't find his courage until very late in the game. Mann is the champ of moralistic revisionism and over-the-shoulder judgments, of proclaiming with piety now how people in another time behaved or should have behaved.

Much later, Mann drops coy hints of a conspiracy by Memphis authorities - and, implicitly, the FBI - in the assassination of Dr. King. A documentary-style drama on prime-time network television does not seem just the forum for raising such speculation, especially is a surreptitious, half-whispered way.

Mann is a slickly skillful dramatist, however, and "King" might have matched the riveting impact of "Roots," if only the script had been shaped by a competent director. It wasn't. The film was directed by Mann himself and it's safe to say that the director and the author managed to bring out some of the worst in each other.

Worse, Mann has found new ways to abuse the already questionable tactics of the TV docu-drama. Not only is actual newsreel footage intercut with the speculative narrative, but occasional new sense were shot in black and white and with hand-held cameras so that they would look like newsreels, too.

Julian Bond, Ramsey Clark and, of all people, Singer Tony Bennett pop up as themselves, and this seems a gambit to bolster the illusion of anthenticity, and a disreputable one of that.Mann was clearly in a messianic mood, but at times he seems more concerned with advertising his own concern, and his own real or imagined credibility, than in telling King's story faithfully and movingly.

Dubious emphasis plagues the film. The role of Coretta Scott King in the movements and in shaping King's role may have been overstressed. Mrs. King was paid a fee for her cooperation in the filming, according to Mann, and had approval rights over the script and the casting. Mrs. King later wrote an article about the film for TV Guide magazine.

After an opening scene of riots in Memphis prior to King's assassination, Mann flashes back to the first time King met his future wife, and this is presented as, in effect, the beginning of the civil rights movement. Meanwhile, such figures as Ralph Abernathy, who was part of the movement but did not cooperate with the filming, remain on the silent periphery; some fictitious characters were added in the interests of good storytelling.

Again the conflict between good storytelling and accurate history arises. "King" is not an exploitative insult, like the recent "Ruby and Oswald" on CBS, but it tinkers about with realities that are strong enough and certainly important enough to have been presented in less gimmicky and obtuscating ways.

To the credit, perhaps the salvation, of "King," the title role is played with striking empathy and consistent conviction by Paul Winfield. It couldn't have been easy to make a man so legendary and so widely idolized - however widely derided he also was in his time - as believable and life-sized as this. Winfield is outstanding.

It is only during the oratorical scenes that his performance proves inadequate, because the speeches of Martin Luther King were delivered with a penetrating force and theatrical flourish that make them unforgettable. Those of us who lived the era through television can close our eyes and hear the "I have a draem" speech just as he delivered it, so that Winfield's version is doomed to fail by comparison, and does.

Mann directs the speech scenes so poorly, anyway, that Winfield is repeatedly undercut just when he's working up some real evangelical steam. Standing ovations occur at the oddest and most disruptive moments and Mann is obsessed with showing us this audience reaction.

There's so much random applause it begins to look like a Dean Martin Roast. Perhaps Mann could only relate, to King's success with crowds in show-biz terms; it would figure.

Cicely Tyson brings nobility without piety to the role of Mrs. King, though it is rough going for both her and Winfield during Mann's sloppy and unseemly courtship scenes. The quality of other performances varies but two exceptional actors stand out: Howard Rollins, who is sensational as Andrew Young - or the version of Andrew Young that Mann created with Young's approval - and Kenneth McMillan, who is brilliantly terrifying as police chief "Bull" Connor.

John and Robert Kennedy are shown watching the Birmingham demonstrations on television. Mann's dialogue makes them sound preoccupied with political consequences only and, therefore, within the context of "King," slightly dishonorable. But King was also a politician, and a skillful one, and it is specious for King to state that politics is morally suspect.

At this point in the film, Mann had President Kennedy saying, "Imagine the b-s it takes to try to integrate Birmingham." Mann insists the line is authentic and told to him by Bobby Kennedy some time later. Authentic or not, the NBC censor has since ordered it deleted.

Advance criticsm of "King" from Abernathy contended that Mann portrayed King as dependent on the leadership and beneficence of certain noblesse obige whites. A scene in "King" supports his charge; one of the few times Mann has King breaking into tears is when King has to dismiss a white lawyer from the movement because the FBI has found a Communit skeleton in the man's closet.

The Tony Bennett scenes, narrated by Bennett, seem to say that racial violence in the South only really got horrible when it was even directed against sympathetic whites. At best the scenes are ludicrous and gratuitous.

At a press conference that followed a screening of "King," in unfinished form, last month in Los Angeles, Mann defended such touches. "Tony is a personal friend," he said of the Bennett appearance. "I thought it would be poignant. He represents hundreds of entertainers who did that (performed at civil rights rallies in the South)."

Mann called the $5-million production "my spiritual love letter" to King because, "To me, he's really our greatest American." He defended the portrayals of John and Robert Kennedy and said they were not meant to be derogatory. Of Bobby, Mann said, "To me, he is a man who grows," and of Jack, "He was a man who grew."

The 64 question, though, was why mann, who did not have directorial experience, directed the film himself instead of hiring someone better equipped to do it. Mann gave that one a $64 answer: "I don't want to name-drop," he said, "but it was Cary Grant who suggested it."

But the fact remains that the subject of "King" has such strength and still carries such meaning that the film's many shortcomings often seem unimportant.

Mann insists that his film is "far more explosive than anything you'll see in any feature motion picture this year or for many years," and although that's just more Hollywood hooey. "King" is uncommonly and undeniably powerful television - a harrowing account of how difficult it was to effect monumental social change through nonviolent means and to become, in the process, one of the most influential pacifists in history.

It is a great story, and someday a great storyteller may tell it.