Chappaquiddick. The word tumbles out with a cadence all its own, like a cantata that carries an undercurrent of timpani. The sound is at once familiar, yet somehow foreboding; precise, yet somehow undefined.

Chappaquiddick. After more than 10 years, it has been thrust upon the national conscience once again, as Sen. Edward M. Kennedy, after three flirtations in past presidential years, has declared his candidacy for president.

It is, despite having been part of the lore about our leaders and ourselves for more than a decade, still an ephemeral thing -- in a sense, a very personal thing. Chappaquiddick is the name of a small place that, like Watergate, has come to signify a large national scanda. But while Watergate came, to all, to mean a series of sweeping but commonly acknowledged criminal acts -- burglary, bugging and the historic cover-up that was obstruction of justice -- Chappaquiddick remains a vague catchall of tragedy that is defined according to the curious mix of passions, prejudices and politics within us all.

There is agreement only that it embraces events that are tragic and actions that are, as Kennedy concedes, inexcusable.

Some think first of Chappaquiddick in terms of prurience and morality,ity, which is to say sex. A married senator was out with a single young woman, and that is what bothers them most.

Some see it in terms of an automobile tragedy: a human being died while the senator, inexplicably, did not contact the authorities in time to attempt to save her.

Some see it in terms of a cover-up of ghastly proportions, despite Kennedy's firm denials, in which the facts seem to indicate that the senator was constructing an alibi: He would pretend that he knew nothing of the accident, rather than contact rescue authorities and thereby admit guilt.

Now, in the presidential campaign for 1980, we are confronted with the word and its myriad meanings that play at least as heavily upon our emotions as upon our politics.

Twice since the accident at Chappaquiddick, the voters of Massachusetts have reelected Kennedy as their senator. But those elections of 1970 and 1976, in a state that long ago took the Kennedys to its heart, provide no true measure of the national impact of the incident. Nor can Kennedy partisans take true comfort in the fact that the legend of Chappaquiddick has remained largely unaltered in the decade that has passed.

Chappaquiddick is, politically, a ticking time bomb. The nation saw in the scandal of Watergate that those who take comfort in the adage that time heals all wounds learn that it also can work the other way. Richard Nixon could win reelection in a landslide, only to quickly fall victim to subsequent revelations and wind up being politically buried by his own cover-up. Only Kennedy and his confidants of that night in 1969 know if there is a cover-up that can be unearthed by reexamining the explanations that they concede are implausible yet insist are true.

The resifting has begun.

Two national television networks, CBS and ABC, have given us dramatic and thought-provoking interviews with Kennedy on the scandal that is with him always.

Major newspapers are presenting lengthy analyses of the Chappaquiddick incident. In the nation's capital, The Washington Star put out a special section on it a week ago Sunday, just before Kennedy announced his candidacy; The Washington Post presented a two-part Chappaquiddick analysis last Sunday and yesterday.

It would seem to some that there is a Kennedy strategy behind all this, a strategy on the politics of Chappaquiddick that has Kennedy people plotting to get discussion of the scandal over with early, in the hope that its impact will diminish before theh first ballots of 1980 are cast. But those closest to Kennedy say it did not happen that way, and in this they seem to have logic on their side.

Kennedy had always known that if he ran for president, he would be asked about Chappaquiddick, his advisers say. The strategic decision came down to this: Kennedy concluded long ago, according to these advisers, that there was no way he could run and refuse to answer questions on the scandal. He would have to respond to them when they were asked. And that, they say, is the beginning and the end of what has seemed to be strategy of a get-it-over-with-early.

"The choice of timing was dictated by the media, not by us," says Thomas Southwick, Kennedy's press secretary. "His only decision was that he will be responsive to Chappaquiddick whenever it is brought up."

Another senior adviser to Kennedy adds: "The Kennedys and the people around the Kennedys are always credited with designing grand schemes. But there just wasn't a grand strategy for this one and there couldn't be one. bIt was clear the questions would come and that he would have to answer them as they came. That's what's been going on."

That is what makes the political impact of Chappaquiddick so difficult to assess. Those running the campaign for Kennedy and those running the campaigns against him say they are unable to judge how the tragedy will affect public opinion.

Others who make public opinion their business are equally unsure.

In a conference room on the ninth floor of a building in downtown Washington, the takers of the public pulse have assembled to talk of politics and polling.

They are far from a random sampling of their industry. They are the dominant opinion-makers of their profession, gathered for a seminar under the aegis of the National Council on Public Polls.

Moving through the crowd of pollsters, a reporter surveys the experts on this question: How will Chappaquiddick affect the 1980 campaign for the presidency?

The response is unanimous. The experts say they do not know.

From George Gallup Sr., the dean of the national corps, to Mervin Field, conductor of the respected California Poll, to Peter D. Hart, who hopes to be Kennedy's pollster, to Patrick Caddell, who is already President Carter's, all say there is no way of measuring with any scientific accuracy how Chappaquiddick would affect voting today, let alone how it will come to affect voting on election day. (Caddell was the only expert who did not want to discuss at length the question of Chappaquiddick and public opinion, being sensitive to the weight of his Carter connection.)

A number of polls have tried to deal with the Chappaquiddick question in a straightforward manner, and generally they have found that anywhere from 20 to 25 percent of respondents say Chappaquiddick would influence them to vote against Kennedy. But all of the public opinion experts concede that these questions don't adequately test voter motivation.

For example, many of those who say Chappaquiddick would influence them to vote against Kennedy may be generally anti-Kennedy anyway, and Chappaquiddick just reinforces their inclination.

A New York Daily News poll conducted by telephone last month found 25 percent of the public saying Chappaquiddick would influence them to vote against Kennedy. But of those who so responded, 19 percent were Democrats and 42 percent were Republicans. And there is no way of knowing how many of these were staunchly anti-Kennedy, anyway.

Some Kennedy partisans have taken comfort from the polling numbers, figuring that if somewhere between 75 and 80 percent are saying they are not bothered by Chappaquiddick enough to vote against Kennedy because of it, the issue must be fading away. They hope that once the initial round of Chappaquiddick stories are done the issue will disappear.

But Peter Hart, who has been mentioned frequently by Kennedy advisers as the pollster who may wind up doing Kennedy campaign surveys, cautions that the existing data may tell little of the national electoral impact of Chappaquiddick.

"Nobody has measured the issue correctly," Hart says, and he includes his organization in that assessment. "All of the polls on the matter have asked very straightforward questions. To really understand the impact of Chappaquiddick, it is necessary to ask a series of questions that are much more oblique."

Hart's point is that people who are swayed negatively by Chappaquiddick may be unwilling to come right out and admit this, even though it could be a factor in causing them to vote against Kennedy.

Hart says he dealt with this problem of subtle bias when he was polling for Ella Grasso during her campaign for governor of Connecticut. Grasso wanted to know if people had a bias against electing a woman governor. Hart devised a questionnaire that began by stating that often people feel some government positions are better held by men and others by women, then asked their views on jobs including FBI director, attorney general, secretary of state, Highway Patrol commander, consumer affairs director and governor.

"We tried to get rid of people's reluctance to discuss biases they might have," Hart says. "We tried to say to them, in effect, it is all right to feel that a man can do the job better."

(Hart's poll found that 39 percent believed it would be better if the governor were a man, which was bad news for Grasso at the time but is a bit easier for her to take now, because she won the election.)

Hart says similar oblique questioning techniques must be used to measure the impact of Chappaquiddick fully, and his colleagues, including Gallup, agree.

"You really can't determine how this Chappaquiddick issue affects people from what has been done so far," Gallup says. "And it is even harder to understand what may happen as the campaign goes on. My guess is that we won't know until we see how the media handles it -- and until the Republicans come back from the hustings much later with a good sense of whether it is turning people on, getting them stirred up. If they think it is, they will make Chappaquiddick into an issue -- an issue of leadership. If not, then it may prove to have only minimal effect."

In the initial days of the Kennedy campaign, Chappaquiddick has at least become an issue for discussion in the media. There has been some commentary as well, perhaps the most notable in the Wall Street Journal, which devoted two page-length columns to an editorial entitled "Chappaquiddick and Credibility," complete with annotated photographs of the paved road Kennedy thought he was on and the dirt road he actually was on, having turned sharply to the right to get there.

Accompanying the editorial was a page-long column of the judge's inquest report, which concluded that Kennedy was not telling the truth. The editorial questioned whether voters "can believe his account of the major crisis in his life, and whether they could believe what he would tell them about any crisis of his presidency."

So, too, there have been more strident media attacks on Kennedy. There were the full page ads in The Washington Post and The Boston Globe last week, purchased at a total cost of $24,000 by a far-right group known as the National Conservative Political Action Committee. "QUESTIONS THE MEDIA WON'T ASK TED KENNEDY" the headline secreamed. Six subjects were presented: four seeking to paint Kennedy as a liberal, one about leadership, and one about "numerous inconsistencies" regarding Chappaquiddick.

The ads were so shrill, so virulently hardline conservative, that it is likely they will provoke only sympathy from anyone who ever remotely considered voting for Kennedy. In contrast to the tone and typeface of the ads, the man behind them talks quietly and thoughtfully about Chappaquiddick as a political issue.

"Will Chappaquiddick be a significant campaign issue in 1980? I guess my answer is that I don't know," says John T. (Terry) Colan, chairman of the committee. "But there is one way to see that it is not an issue. That is for Kennedy to agree to take a lie detector test. If he says yes, and then he passes the test, then the issue is dead."

The CBS television interview with Kennedy a week ago received much public attention as many of Kennedy's responses to probing questions from Roger Mudd were halting, funbling and unconvincing.

As he tried to extricate himself from an inconsistency in his story, Kennedy cast further doubt upon all of his denials that he had tried to construct an alibi for himself.

Millions were listening and watching as Kennedy spoke of his concern for Mary Jo Kopechne shortly after the accident. It was 12:15 a.m., he says, and he was sure because ". . . I was very conscious of the time, at that moment. Because I knew time was ticking by . . . I sensed internally that it would be difficult to expect that you'd have a sense of rescue. But I knew that moments were important. . . . "

Yet everyone is now aware that Kennedy did not call the rescue authorities, not even when he went back to his own motel, changed into dry clothes and engaged the motel clerk in small talk, acting unconcerned and unaware of any accident. He did not call that night -- he went to sleep instead. It is a point that sticks in the throats of even Kennedy's most loyal and enthusiastic supporters. It is part of the political problem that will not go away.

Chappaquiddick. The word will surface from time to time as the 1980 campaign moves on. There will be some placards and some hecklers and more reporters with more questions.

Kennedy says he will continue to respond to all questions. The political reality is that he has no choice. But as Kennedy reluctantly responds, he and those around him nurture the hope that he will be believed and forgiven, and that the foreboding timpani of Chappaquiddick will fade into the past.

"People will have to make that judgment," Kennedy says now. It is his stock response, delivered somberly, as he asks the public to forgive his actions that night and embrace his candidacy a decade later.

EPILOGUE: The Kennedy adviser is talking about Chappaquiddick and whether the new round of interviews will encourage people to believe the senator's explanation.

"I don't know," he says. "Maybe not. After all, it's not that the questions haven't all been asked before. They've all been asked and all been answered. It's that people don't like the answers."