In the living room of Jacqueline Kennedy Onassis' bungalow at Hyannis Port, members of the immediate and extended families of Edward M. Kennedy drew their chairs into a circle and faced the prospect of yet another presidential siege.

Some who gathered on the day after Thanksgiving were veterans of decades of clan strategy summits in behalf of Kennedys past. Others were of the new generation -- the Robert F. Kennedys, for example, were represented by their eldest children, Joseph and Kathleen.

This time, the clan had convened to hear the assessment of a member of a new generation of Kennedy campaign manager, Lawrence Horowitz, 37, who figured to run Kennedy's 1984 presidential campaign and who had sneaked away from Kennedy's Senate staff office unbeknownst to his colleagues there to make this three-hour presentation.

What he had to say, it turned out, did not matter at all. Horowitz distributed a series of summary sheets to bolster the case of the political advisers that, on the politics of it, Kennedy should run and could win.

He presented preliminary test data compiled by pollster Patrick Caddell showing that Kennedy's highly expensive and intensely personal series of ads had switched people from the belief that Kennedy was immoral to the opinion that he was a moral man.

He distributed economic analyses from famous experts that said the economy looked bad through 1984, with unemployment remaining high and a recovery mild at best.

He was well into his analysis when one of the younger generation of Kennedys interrupted:

"I'm not most concerned about the poll data; I'm concerned about what it is going to do to us."

The senator recalls that it was his son, Teddy, who made the comment; Horowitz recollects that it was one of Robert Kennedy's children. Either way, both agree, it pointed the direction of the decision to come.

As the Kennedys and their closest associates told it yesterday, it was the strong and persistent objections of the senator's three children that persuaded him not to run. It was done for the sake of the Kennedy children, most of all for the youngest, 15-year-old Patrick, they say.

And although they have all heard the widely held view that Kennedy could not win a presidential election -- and that he would never overcome those personal character problems that have followed him since Chappaquiddick -- they say that was not their view at all.

The political judgment, they maintain, was that Kennedy was well organized and able to overcome those ever-present personal concerns.

Kennedy had come to Hyannis Port for Thanksgiving fresh from a post-election vacation in Europe, knowing that his children had expressed concern about his running for president, but not knowing the intensity of their views.

Before leaving for Europe, he had personally leaned hard on former senator Harold Hughes of Iowa and succeeded in convincing him to abandon his support of the presidential candidacy of longtime friend Sen. Alan Cranston (D-Calif.), also a longtime friend of Hughes', and to switch to Kennedy--on the basis, Kennedy had argued, that he could win and Cranston could not.

Also, just before he left, he had authorized a new and costly round of polling in Iowa and Illinois by two prominent Democratic pollsters other than Caddell.

When Kennedy called the clan to order in the living room of Jacqueline Kennedy Onassis' home (she was not there), he had not received the results of those new polls, nor would he by the time he made his final decision.

Seated in a circle were: Kennedy brother-in-law Stephen Smith, who had figured prominently in all of the campaigns of Kennedys past and was Sen. Kennedy's 1980 campaign manager; the Kennedy sisters, Jean Kennedy Smith and Patricia Kennedy Lawford; the three children of Sen. Kennedy and the two children of Robert Kennedy, and the Smiths' son, Stephen Jr.

They paid particular attention to the Caddell testing of the effect of those four Senate campaign ads, each a whopping five minutes long, dealing with Kennedy's personal travails and depicting him as a man who is compassionate but "not a plaster saint." What he presented was only part of the overall Caddell survey -- the final analysis is not yet completed.

It showed significant changes when the ads were seen by individuals in New Hampshire, the first primary state, which is served by Massachusetts stations.

Before seeing the ads, the ratio of those believing Kennedy was moral to those believing him immoral was 35 to 49. After seeing the ads, the ratio switched to 52 to 35 -- a change of 31 points in Kennedy's favor.

Before seeing the ads, more people thought Kennedy untrustworthy than trustworthy; this too was reversed after the ads were seen, for a gain of 15 points in Kennedy's favor. Asked if they thought Kennedy panicked in a crisis, many more people said yes than no before seeing the ads; after viewing them, more still said they thought he did panic than didn't -- but the margin was reduced by 17 points.

The criticisms of Kennedy's personal character had hit the family hard in the 1980 campaign.

After the meeting of the clan had ended, Kennedy began what would be several hours of talks with his children, Kara, Ted Jr. and Patrick. On Sunday, after the last of those meetings, Kennedy told Horowitz that the children were unanimous in their feeling that he should not run. They had argued that 1980 was different; now, with the pending divorce of their parents they were worried about their father's security and felt the need to keep the family together at all costs -- particularly for the sake of Patrick.

The next day, Kennedy met with his political advisers and went over it all again. He felt he could win, but he felt he could not run. "Nothing will change my mind," he told his advisers Tuesday morning in Washington. "So let's get it over with."

When the ordeal of the press conference was over, the Kennedy children talked buoyantly of the ambition that their father had abandoned. "I made the decision a long time ago," said Teddy, 21. "I didn't want him to run . . . for Patrick, mostly. When we are all grown up, it will be different."

"It was our decision as a family," said Patrick. "And I think he made the right decision. I mean, he had some obligations to the family . . . "