The Kennedy Legacy

The images have been burned into unforgettable memories for half a century; the young senator who lost the battle for the vice-presidency in 1956 - then won his party's presidential nomination four years later, CBS News senior political correspondent Jeff Greenfield.

"We stand today at the edge of a New Frontier. The frontier of the 1960's a frontier of unknown opportunities and perils - a frontier of unfulfilled hopes and threats," said John F. Kennedy in 1960.

Those images include a memorable inaugural, at which he said: "Ask not what your country can do for you; ask what you can do for your country."

And a White House impossibly young and glamorous. Then there are the other ineradicable memories.

It was at the 1964 Convention, Robert Kennedy paid tribute to his fallen brother John.

"I realize that as an individual even more importantly, for our political party and for the country that we can't just look to the past, but we must look to the future," Robert Kennedy said.

Four years later, Ted Kennedy was paying tribute to the murdered Robert.

In 1969, came the death of a young woman in a car accident with Ted Kennedy at the wheel.

Lingering memories of that helped doom Ted Kennedy's 1980 effort to take the Democratic nomination away from the President Carter.

But at that convention, Kennedy's speech became a rallying cry for the liberal foot soldiers of the party.

"The work goes on, the cause endures, the hope still lives, the dream shall never die," he said.

But beyond the personal triumphs and losses, there is another story - the political legacy of the Kennedys that began a generation earlier than most of us realize, and that shaped, and reflects, the modern history of the Democratic Party.

It was Joseph Kennedy, Sr. - the patriarch of the clan - whose political clout was crucial to FDR's nomination in 1932, without which there would have been no New Deal.

It was John Kennedy's presidency that attracted a legion of the young into politics. And it was Robert Kennedy who spoke to the passionate divisions of the late 1960s.

And it was Ted Kennedy who became the Senate's liberal lion, leading fights for an expansive federal role even as the country moved to the Right.

"The younger brothers Bobby and Ted were infected by one particular aspect of John, which was his idealism … and Bobby was the suffering idealist. And Ted became over time the pragmatic idealist," said Joe Klein, a columnist with Time Magazine.

But now the Kennedy presence is fading: Caroline Kennedy helped vet Obama's vice-presidential pick, but has no apparent political ambitions. Ted's son, Rhode Island Congressman Patrick Kennedy, is the only third-generation Kennedy holding office. And Ted Kennedy is in a battle with a life-threatening illness.

"In terms of electoral politics, in terms of having another Kennedy presidency I think that dream is done," said Klein.

Monday, once again, a Democratic convention will pay tribute to a Kennedy, but you can't help wondering if this celebration will be something of a farewell salute to one of the most prominent political families in American history.