Senator Edward M. Kennedy of Massachusetts, a son of one of the most storied families in American politics, a man who knew acclaim and tragedy in near-equal measure and who will be remembered as one of the most effective lawmakers in the history of the Senate, died late Tuesday night. He was 77.

The death of Mr. Kennedy, who had been battling brain cancer, was announced Wednesday morning in a statement by the Kennedy family, which was already mourning the death of the senator’s sister Eunice Kennedy Shriver two weeks earlier.

“Edward M. Kennedy — the husband, father, grandfather, brother and uncle we loved so deeply — died late Tuesday night at home in Hyannis Port,” the statement said. “We’ve lost the irreplaceable center of our family and joyous light in our lives, but the inspiration of his faith, optimism and perseverance will live on in our hearts forever.”

President Obama said Mr. Kennedy was one of the nation’s greatest senators.

“His ideas and ideals are stamped on scores of laws and reflected in millions of lives — in seniors who know new dignity, in families that know new opportunity, in children who know education’s promise, and in all who can pursue their dream in an America that is more equal and more just — including myself,” he said. Mr. Obama is scheduled to speak at a funeral Mass for Mr. Kennedy on Saturday morning in Boston.

Mr. Kennedy had been in precarious health since he suffered a seizure in May 2008. His doctors determined the cause was a malignant glioma, a brain tumor that carries a grim prognosis.

As he underwent cancer treatment, Mr. Kennedy was little seen in Washington, appearing most recently at the White House in April as Mr. Obama signed a national service bill that bears the Kennedy name. In a letter last week, Mr. Kennedy urged Massachusetts lawmakers to change state law and let Gov. Deval Patrick appoint a temporary successor upon his death, to assure that the state’s representation in Congress would not be interrupted.

While Mr. Kennedy was physically absent from the capital in recent months, his presence was deeply felt as Congress weighed the most sweeping revisions to America’s health care system in decades, an effort Mr. Kennedy called “the cause of my life.”

On July 15, the Senate Health, Education, Labor and Pensions Committee, which Mr. Kennedy headed, passed health care legislation, and the battle over the proposed overhaul is now consuming Capitol Hill.

Mr. Kennedy was the last surviving brother of a generation of Kennedys that dominated American politics in the 1960s and that came to embody glamour, political idealism and untimely death. The Kennedy mystique — some call it the Kennedy myth — has held the imagination of the world for decades, and it came to rest on the sometimes too-narrow shoulders of the brother known as Teddy.

Mr. Kennedy, who served 46 years as the most well-known Democrat in the Senate, longer than all but two other senators, was the only one of those brothers to reach old age. President John F. Kennedy and Senator Robert F. Kennedy were felled by assassins’ bullets in their 40s. The eldest brother, Joseph P. Kennedy Jr., died in 1944 at the age of 29 while on a risky World War II bombing mission.

Mr. Kennedy spent much of the last year in treatment and recuperation, broken by occasional public appearances and a dramatic return to the Capitol last summer to cast a decisive vote on a Medicare bill.

He electrified the opening night of the Democratic National Convention in Denver in August with an unscheduled appearance and a speech that had delegates on their feet. Many were in tears.

His gait was halting, but his voice was strong. “My fellow Democrats, my fellow Americans, it is so wonderful to be here, and nothing is going to keep me away from this special gathering tonight,” Mr. Kennedy said. “I have come here tonight to stand with you to change America, to restore its future, to rise to our best ideals and to elect Barack Obama president of the United States.”

Senator Kennedy was at or near the center of much of American history in the latter part of the 20th century and the early years of the 21st. For much of his adult life, he veered from victory to catastrophe, winning every Senate election he entered but failing in his only bid for the presidency; living through the sudden deaths of his brothers and three of his nephews; being responsible for the drowning death on Chappaquiddick Island of a young woman, Mary Jo Kopechne, a former aide to his brother Robert. One of the nephews, John F. Kennedy Jr., who the family hoped would one day seek political office and keep the Kennedy tradition alive, died in a plane crash in 1999 at age 38.

Mr. Kennedy himself was almost killed in 1964, in a plane crash that left him with permanent back and neck problems.

He was a Rabelaisian figure in the Senate and in life, instantly recognizable by his shock of white hair, his florid, oversize face, his booming Boston brogue, his powerful but pained stride. He was a celebrity, sometimes a self-parody, a hearty friend, an implacable foe, a man of large faith and large flaws, a melancholy character who persevered, drank deeply and sang loudly. He was a Kennedy.

Senator Robert C. Byrd, Democrat of West Virginia, one of the institution’s most devoted students, said of his longtime colleague, “Ted Kennedy would have been a leader, an outstanding senator, at any period in the nation’s history.”

Mr. Byrd is one of only two senators to have served longer in the chamber than Mr. Kennedy; the other was Strom Thurmond of South Carolina. In May 2008, on learning of Mr. Kennedy’s diagnosis of a lethal brain tumor, Mr. Byrd wept openly on the floor of the Senate.

More Than a Legislator

Born to one of the wealthiest American families, Mr. Kennedy spoke for the downtrodden in his public life while living the heedless private life of a playboy and a rake for many of his years. Dismissed early in his career as a lightweight and an unworthy successor to his revered brothers, he grew in stature over time by sheer longevity and by hewing to liberal principles while often crossing the partisan aisle to enact legislation. A man of unbridled appetites at times, he nevertheless brought a discipline to his public work that resulted in an impressive catalog of legislative achievement across a broad landscape of social policy.

Mr. Kennedy left his mark on legislation concerning civil rights, health care, education, voting rights and labor. He was chairman of the Senate Committee on Health, Education, Labor and Pensions at his death. But he was more than a legislator. He was a living legend whose presence ensured a crowd and whose hovering figure haunted many a president.

Although he was a leading spokesman for liberal issues and a favorite target of conservative fund-raising appeals, the hallmark of his legislative success was his ability to find Republican allies to get bills passed. Perhaps the last notable example was his work with President George W. Bush to pass No Child Left Behind, the education law pushed by Mr. Bush in 2001. He also co-sponsored immigration legislation with Senator John McCain, the 2008 Republican presidential nominee. One of his greatest friends and collaborators in the Senate was Orrin G. Hatch, the Utah Republican.

Mr. Kennedy had less impact on foreign policy than on domestic concerns, but when he spoke, his voice was influential. He led the Congressional effort to impose sanctions on South Africa over apartheid, pushed for peace in Northern Ireland, won a ban on arms sales to the dictatorship in Chile and denounced the Vietnam War. In 2002, he voted against authorizing the Iraq war; later, he called that opposition “the best vote I’ve made in my 44 years in the United States Senate.”

At a pivotal moment in the 2008 Democratic presidential primaries, Mr. Kennedy endorsed Mr. Obama, then an Illinois senator, Obama for president, saying he offered the country a chance for racial reconciliation and an opportunity to turn the page on the polarizing politics of the past several decades.

“He will be a president who refuses to be trapped in the patterns of the past,” Mr. Kennedy said at an Obama rally in Washington on Jan. 28, 2008. “He is a leader who sees the world clearly, without being cynical. He is a fighter who cares passionately about the causes he believes in without demonizing those who hold a different view.”

This month, Mr. Obama awarded Mr. Kennedy the Presidential Medal of Freedom, which his daughter, Kara, accepted on his behalf.

Mr. Kennedy struggled for much of his life with his weight, with alcohol and with persistent tales of womanizing. In an Easter break episode in 1991 in Palm Beach, Fla., he went out drinking with his son Patrick and a nephew, William Kennedy Smith, on the night that Mr. Smith was accused of raping a woman. Mr. Smith was prosecuted in a lurid trial that fall but was acquitted.

Mr. Kennedy’s personal life stabilized in 1992 with his marriage to Victoria Anne Reggie, a Washington lawyer. His first marriage, to Joan Bennett Kennedy, ended in divorce in 1982 after 24 years.

Senator Kennedy served as a surrogate father to his brothers’ children and worked to keep the Kennedy flame alive through the Kennedy Library in Boston, the Kennedy Center in Washington and the Kennedy School of Government at Harvard University, where he helped establish the Institute of Politics.

In December, Harvard granted Mr. Kennedy a special honorary degree. He referred to Mr. Obama’s election as “not just a culmination, but a new beginning.”

He then spoke of his own life, and perhaps his legacy.

“We know the future will outlast all of us, but I believe that all of us will live on in the future we make,” he said. “I have lived a blessed time.”

Kennedy family courtiers and many other Democrats believed he would eventually win the White House and redeem the promise of his older brothers. In 1980, he took on the president of his own party, Jimmy Carter, but fell short because of Chappaquiddick, a divided party and his own weaknesses as a candidate, including an inability to articulate why he sought the office.

But as that race ended in August at the Democratic National Convention in New York, Mr. Kennedy delivered his most memorable words, wrapping his dedication to party principles in the gauzy cloak of Camelot.

“For me, a few hours ago, this campaign came to an end,” Mr. Kennedy said in the coda to a speech before a rapt audience at Madison Square Garden and on television. “For all those whose cares have been our concern, the work goes on, the cause endures, the hope still lives and the dream shall never die.”

A Family Steeped in Politics

Born Feb. 22, 1932, in Boston, Edward Moore Kennedy grew up in a family of shrewd politicians. Both his father, Joseph P. Kennedy, and his mother, the former Rose Fitzgerald, came from prominent Irish-Catholic families with long involvement in the hurly-burly of Democratic politics in Boston and Massachusetts. His father, who made a fortune in real estate, movies and banking, served in President Franklin D. Roosevelt’s administration, as the first chairman of the Securities and Exchange Commission and then as ambassador to Britain.

There were nine Kennedy children, four boys and five girls, with Edward the youngest. They grew up talking politics, power and influence because those were the things that preoccupied the mind of Joseph Kennedy. As Rose Kennedy, who took responsibility for the children’s Roman Catholic upbringing, once put it, “My babies were rocked to political lullabies.”

When Edward was born, President Herbert Hoover sent Rose a bouquet of flowers and a note of congratulations. The note came with 5 cents postage due; the framed envelope is a family heirloom.

It was understood among the children that Joseph P. Kennedy Jr., the oldest boy, would someday run for Congress and, his father hoped, the White House. When Joseph Jr. was killed in World War II, it fell to the next oldest son, John, to run. As John said at one point in 1959 while serving in the Senate: “Just as I went into politics because Joe died, if anything happened to me tomorrow, Bobby would run for my seat in the Senate. And if Bobby died, our young brother, Ted, would take over for him.”

Although surrounded by the trappings of wealth — stately houses, servants and expensive cars — young Teddy did not enjoy a settled childhood. He bounced among the family homes in Boston, New York, London and Palm Beach, and by the time he was ready to enter college, he had attended 10 preparatory schools in the United States and England, finally finishing at Milton Academy, near Boston. He said that the constant moving had forced him to become more genial with strangers; indeed, he grew to be more of a natural politician than either John or Robert.

After graduating from Milton in 1950, where he showed a penchant for debating and sports but was otherwise an undistinguished student, Mr. Kennedy enrolled in Harvard, as had his father and brothers.

It was at Harvard, in his freshman year, that he ran into the first of several personal troubles that were to dog him for the rest of his life: He persuaded another student to take his Spanish examination, got caught and was forced to leave the university.

Suddenly draft-eligible during the Korean War, Mr. Kennedy enlisted in the Army and served two years, securing, with his father’s help, a post at NATO headquarters in Paris. In 1953, he was discharged with the rank of private first class.

Re-enrolling in Harvard, he became a more serious student, majoring in government, excelling in public speaking and playing first-string end on the football team. He graduated in 1956 with a Bachelor of Arts degree, then enrolled in the University of Virginia School of Law, where Robert had studied. There, he won the moot court competition and took a degree in 1959. Later that year, he was admitted to the Massachusetts bar.

Mr. Kennedy’s first foray into politics came in 1958, while still a law student, when he managed John’s Senate re-election campaign. There was never any real doubt that Massachusetts voters would return John Kennedy to Washington, but it was a useful internship for his youngest brother.

That same year, Mr. Kennedy married Virginia Joan Bennett, a debutante from Bronxville, a New York suburb where the Kennedys had once lived. In 1960, when John Kennedy ran for president, Edward was assigned a relatively minor role, rustling up votes in Western states that usually voted Republican. He was so enthusiastic about his task that he rode a bronco at a Montana rodeo and daringly took a ski jump at a winter sports tournament in Wisconsin to impress a crowd. The episodes were evidence of a reckless streak that repeatedly threatened his life and career.

John Kennedy’s election to the White House left vacant a Senate seat that the family considered its property. Robert Kennedy was next in line, but chose the post of attorney general instead (an act of nepotism that has since been outlawed). Edward was only 28, two years shy of the minimum age for Senate service.

So the Kennedys installed Benjamin A. Smith II, a family friend, as a seat-warmer until 1962, when a special election would be held and Edward would have turned 30. Edward used the time to travel the world and work as an assistant district attorney in Boston, waiving the $5,000 salary and serving instead for $1 a year.

As James Sterling Young, the director of a Kennedy Oral History Project at the University of Virginia, said the catchphrase of that era was: “Most people grow up and go into politics. The Kennedys go into politics and then they grow up.”

Less than a month after turning 30 in 1962, Mr. Kennedy declared his candidacy for the remaining two years of his brother’s Senate term. He entered the race with a tailwind of family money and political prominence. Nevertheless, Edward J. McCormack Jr., the state’s attorney general and a nephew of John W. McCormack, then speaker of the United States House of Representatives, also decided to go after the seat.

It was a bitter fight, with a public rehash of the Harvard cheating episode and with Mr. McCormack charging in a televised “Teddy-Eddie” debate that Mr. Kennedy lacked maturity of judgment because he had “never worked for a living” and had never held elective office. “If your name was simply Edward Moore instead of Edward Moore Kennedy,” Mr. McCormack added, “your candidacy would be a joke.”

But the Kennedys had ushered in an era of celebrity politics, which trumped qualifications in this case. Mr. Kennedy won the primary by a two-to-one ratio, then went on to easy victory in November against the Republican candidate, George Cabot Lodge, a member of an old-line Boston family that had clashed politically with the Kennedys through the years.

When Mr. Kennedy entered the Senate in 1962, he was aware that he might be seen as an upstart, with one brother in the White House and another in the cabinet. He sought guidance on the very first day from one of the Senate’s most respected elders, Richard Russell of Georgia. “You go further if you go slow,” Senator Russell advised.

Mr. Kennedy took things slowly, especially that first year. He did his homework, was seen more than he was heard and was deferential to veteran legislators.

On Friday, Nov. 22, 1963, he was presiding over the Senate when a wire service ticker in the lobby brought the news of John Kennedy’s shooting in Dallas. Violence had claimed the second of Joseph Kennedy’s sons.

Edward was sent to Hyannis Port to break the news to his father, who had been disabled by a stroke. He returned to Washington for the televised funeral and burial, the first many Americans had seen of him. He and Robert had planned to read excerpts from John’s speeches at the Arlington burial service. At the last moment they chose not to.

A friend described him as “shattered — calm but shattered.”

Edward M Kennedy