As Sen. Edward M. Kennedy (D-Mass.) sat in an easy chair at his mother's home in Hyannisport, Mass., Wednesday evening, telling a surprised circle of close advisers why he wouldn't be running for president in 1988, political consultant Robert Shrum asked his ex-boss to weigh the consequences of his decision.

"I told him 1988 was going to be the best chance he was going to get," Shrum said.

"He said, 'Well, I realize this means I may never be president.' "

Shrum had never heard Kennedy say that before, and the very notion seemed to strike at the heart of the first imperative of being a Kennedy.

Yet the burden of that family heritage, and the strong personal and political urge to be rid of it, appears to have been what drove Kennedy out of presidential politics -- now and quite likely forevermore, according to friends and advisers.

"It was the lifting of a weight that he has to run for president," Shrum said. "He is the only man in politics who has had to carry that weight."

Relaxed and self-confident, Kennedy said in a brief news conference today that that while he "still would like to be president," he simply is not willing endure a four-year campaign, a campaign that would inevitably be complicated by the "passions" any Kennedy candidacy arouses.

"In 1960, when John F. Kennedy announced for president in January . . . it was considered very early," Kennedy said. "Now, the reality is that the presidential nomination process begins the day after the last election."

The five-term senator said he chafed at the prospect that his every move in the Senate would be judged in terms of presidential positioning. To avoid that, he said he chose to get out early. In addition, aides said, he felt a "moral obligation" not to lead his staff, his fund-raisers and political operatives down a blind alley.

"There is a judgment one makes about how you can be most effective in public life, and as I thought that issue through, I found that the decision was not complicated -- and I'm very satisfied with it," Kennedy said. He added, in response to a question, that he would not accept a draft.

"He is a man who wants to be president, but who doesn't need to be president," Shrum said.

"He is very comfortable with the idea that his career is in the Senate," added Lawrence Horowitz, Kennedy's administrative assistant and the man in whom he confided as he worked through the decision over the past six weeks.

In mid-November, Horowitz said, Kennedy talked with his three children -- Kara, 25, Ted Jr., 24, and Patrick, 18 -- about a potential 1988 race and was convinced that while they would not object, they would prefer he not run.

He said Kennedy weighed his children's reluctance, along with his assessment that getting elected in 1988 would be a roll of the dice, along with his concern about the "cynicism" with which the news media and politicians would view his every move between now and 1988, and concluded he did not want to put up with "the cost of getting there."

Kennedy aides were adamant that the senator had been "genuinely hurt," in Shrum's phrase, that the media had viewed his trip to South Africa through the prism of presidential politicking rather than as a contribution to a human-rights cause. They said his votes this year in favor of the line-item veto and in support of the Gramm-Rudman-Hollings deficit-reduction bill stemmed from genuine conviction, and Kennedy was "frustrated" that they were written off as the moves of a old liberal trying to reposition himself to the center.

"He found himself getting all this extra scrutiny because he's a Kennedy, and he was in a damned-if-you-do, damned-if-you-don't dilemma," said one aide. "If he votes one way, he's the Old Teddy; if he votes another, he's trying to moderate his image."

Kennedy said at the news conference that while he knew a campaign for the nomination would be "hard and difficult," he was "hopeful" he could win it and took "some sense of satisfaction" in polls showing him leading the Democratic pack.

In recent months, however, a number of state chairs and other party leaders have publicly called on Kennedy not to run, suggesting that his time has passed. Kennedy spent part of the summer and fall trying to line up commitments from fund-raisers and operatives. His political aide, William Carrick, said Kennedy was gratified by the response, although others close to Kennedy said he was running into resistance in many quarters.

Any major Kennedy announcement inevitably sets off rounds of political chatter about hidden motives. One popular gossip item in Washington today is that Kennedy, who was divorced from his wife, Joan, in 1981, is planning to marry. Horowitz said he knew of no such plans and added he would be "flabbergasted" if such considerations had played a role in the decision.

Horowitz also discounted speculation that Kennedy now wants to mount a campaign to become Senate majority leader if Democrats recapture the Senate. Kennedy, he said, would prefer to take over chairmanship of one of the two committees he would have the option of leading -- Judiciary or Labor and Human Resources.

Horowitz and legislative director Carey Parker are expected to stay with Kennedy. His political operatives, Carrick and Paul Tully, are thought likely to move to other presidential candidates.