He plunged into the new job with Kennedy enthusiasm. But fate, and the Kennedy recklessness, intervened on July 18, 1969. Mr. Kennedy was at a party with several women who had been aides to Robert. The party, a liquor-soaked barbecue, was held at a rented cottage on Chappaquiddick Island, off Martha’s Vineyard. He left around midnight with Mary Jo Kopechne, 28, took a turn away from the ferry landing and drove the car off a narrow bridge on an isolated beach road. The car sank in eight feet of water, but he managed to escape. Miss Kopechne, a former campaign worker for Robert, drowned.

Mr. Kennedy did not report the accident to the authorities for almost 10 hours, explaining later that he had been so banged about by the crash that he had suffered a concussion, and that he had become so exhausted while trying to rescue Miss Kopechne that he had gone immediately to bed. A week later, he pleaded guilty to a charge of leaving the scene of an accident and was given a two-month suspended sentence.

But that was far from the end of the episode. Questions lingered in the minds of the Massachusetts authorities and of the general public. Why was the car on an isolated road? Had he been drinking? (Mr. Kennedy testified at an inquest that he had had two drinks.) What sort of relationship did Mr. Kennedy and Miss Kopechne have? Could she have been saved if he had sought help immediately? Why did the senator tell his political advisers about the accident before reporting it to the police?

The controversy became so intense that Mr. Kennedy went on television to ask Massachusetts voters whether he should resign from office. He conceded that his actions after the crash had been “indefensible.” But he steadfastly denied any intentional wrongdoing.

His constituents sent word that he should remain in the Senate. And little more than a year later, he easily won re-election to a second full term, defeating a little-known Republican, Josiah A. Spaulding, by a three-to-two ratio. But his heart did not seem to be in his work any longer. He was sometimes absent from Senate sessions and neglected his whip duties. Senator Byrd, of West Virginia, took the job away from him by putting together a coalition of Southern and border-state Democrats to vote him out.

That loss shook Mr. Kennedy out of his lethargy. He rededicated himself to his role as a legislator. “It hurts like hell to lose,” he said, “but now I can get around the country more. And it frees me to spend more time on issues I’m interested in.” Many years later, he became friends with Mr. Byrd and told him the defeat had been the best thing that could have happened in his Senate career.

Turmoil at Home

In the next decade, Mr. Kennedy expanded on his national reputation, first pushing to end the war in Vietnam, then concentrating on his favorite legislative issues, especially civil rights, health, taxes, criminal laws and deregulation of the airline and trucking industries. He traveled the country, making speeches that kept him in the public eye.