BOSTON - Senator Edward Kennedy's death marks the twilight of one of America's most fabled political families, with no heirs to the Kennedy name poised to emerge with the same mix of gravitas, ambition and celebrity.

Kennedy, 77, one of the most effective lawmakers in U.S. history and the brother of assassinated President John F. Kennedy, died late on Tuesday after battling brain cancer.

He died just weeks after his sister, Eunice Kennedy Shriver, who founded the Special Olympics and was a leading advocate for the mentally disabled.

There seems to be no one there to pick up the torch, said Thomas Whalen, a professor of politics at Boston University.

There doesn't seem to be someone in the next generation to carry the load here. Ted Kennedy might be it, he might be the end of the line, said Whalen, author of Kennedy versus Lodge: The 1952 Massachusetts Senate Race about his brother John's first race for the Senate.

Kennedy's cancer diagnosis and then his death have stirred speculation over who might succeed the third-longest serving U.S. senator, and whether a new generation could emerge from under his shadow.

Many younger Kennedys are active in civic life but none on the scale of Ted Kennedy, last of four Kennedy brothers, including John, elected president in 1960 and assassinated in 1963; and Robert, a New York senator whose presidential bid ended with his assassination in 1968.

The eldest brother, Joseph Jr., was killed in World War Two.

Options are limited for another Kennedy to inherit the Senate seat held by the family for nearly five decades. Kennedy's nephew, Joseph, -- the eldest son of Robert Kennedy -- is often cited as a possibility.

He served six terms as a U.S. congressman from Massachusetts and now runs the nonprofit Citizens Energy Corp, which delivers cheap heating oil to the state's poor.

He considered running for governor of Massachusetts several times but always decided against it, including in 1997 when his ex-wife came out with a less-than-flattering book about him.

After deciding not to run again in 2001, he told The Boston Globe: This is not a rejection of politics for me. I just like what I do at Citizens.

Democrats, aware of the deep affection for the Kennedy family, could urge the 56-year-old to seek his uncle's seat.

Under Massachusetts state law, a vacancy in the U.S. Senate forces the Massachusetts governor to call a special election between 145 and 160 days after it becomes official.

Other Kennedys could also keep the legacy alive, although none project the stature of the white-haired Lion of the Senate, widely regarded as one of Washington's most effective dealmakers and communicators with his speeches full of stirring Kennedy imagery, and a rumbling baritone voice.

Other Kennedy torch-bearers include Ted Kennedy's son, 42-year-old Representative Patrick Kennedy, a Democratic congressman from neighboring Rhode Island. He would need to move to Massachusetts if he were to run for his father's seat.
Patrick Kennedy's brushes with controversy could complicate a run for higher office -- most recently in 2006 when he sought help for dependency on prescription drugs after crashing his car into a security barrier in Washington. Just this year he suffered a relapse in his battle with addiction.

UNCERTAIN POLITICAL FORTUNES

Still, when Ted Kennedy was his son's age, in 1974, his own political future looked nearly just as uncertain.

Five years earlier, Mary Jo Kopechne, a young aide, drowned when Kennedy's car plunged off a bridge in Chappaquiddick on a Massachusetts resort island after a night of partying. Moral character questions and whispers of alcoholism dogged Kennedy at the time.

But unlike his son, the late Kennedy's charisma as the last of the Kennedy brothers was such that draft-Teddy drives were a regular feature of presidential elections. He figured at least briefly in every Democratic nomination campaign from 1968 to 1988.

The political fortunes of another family member, Kathleen Kennedy Townsend, 58, the eldest child of Robert Kennedy and a former lieutenant governor of Maryland, foundered in 2002 when she lost a bid for governor.

Caroline Kennedy, 51, daughter of John F. Kennedy, withdrew her bid to fill the Senate seat for New York vacated by Hillary Clinton.

An author and philanthropist, she faced criticism for giving what some found to be vague or inarticulate answers in media interviews. Some also questioned whether Kennedy, who did not bother to vote in a number of elections, would be getting the seat solely because she carried the family name.

The Caroline Kennedy fiasco was pretty damaging to the Kennedy brand, said Julian Zelizer, a professor of politics at Princeton University.

There are others who are untested: Robert F. Kennedy Jr., an environmental activist and lawyer, has never held office.

There is no other person in the family among the younger sons who I think has the gravitas of a Ted Kennedy at this point in terms of identifying with a certain set of principles, liberalism, or having the same television and political charisma, said Zelizer. It does seem the family is fading as a dynasty.

(Editing by Phil Stewart and Cynthia Osterman)