"But it didn't cause him to give up on public life," he said. "He didn't go back to taking it easy as a wealthy, famous man and taking his leisure on the beach. He worked harder than ever because he was a fighter for the causes of his brothers John and Bobby, reaching out to blacks, the poor and unemployed. He became the concerned uncle for the children in his life and showed compassion for people at the bottom of the economic pyramid."
Though the 1980 Democratic presidential nomination eluded him, Kennedy's riveting convention speech became a clarion call for for the liberal wing of the party and the hallmark of his career forward.
"For me, a few hours ago, this campaign came to an end," said Kennedy. "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."
Sorensen credits Kennedy's early endorsement of Democratic candidate Barack Obama as giving political clout to the now president's 2008 victory.
Still, Chappaquiddick caused Kennedy, who had "moved swiftly up the ladder," to have no chance at an official leadership post. He was passed over in 1971 for the Democratic whip position to Sen. Robert Byrd, D-W. Va., and ruled out as a future minority leader, according to Norm Ornstein, a congressional analyst for the American Enterprise Institute.
"It was a set-back, but actually, in some important ways, it helped to make his career as a senator and caused him to rethink other elements of his life and become a workhorse again for policy instead of leadership," he told ABCNews.com.
Chappaquiddick has become "the secondary story of his life," said Ornstein. "There is certainly no question that he had pretty wild elements through some years in his career. He clearly has a stamina level more than anyone else I ever met, and it didn't shrink his ability to do work and shape legislation when partying."
Even his opposition could find little fault with the senator. "It's impossible not to love him," said Ornstein, just before Kennedy's death.
"He's a dynamic, infectiously funny person who just envelops you," he said. "He really's got tremendous personal charm."
At the same time, a new generation of Americans has little or no memory of Chappaquiddick.
"There were so many vicissitudes in his life -- drinking, problems with his first wife, the loss of two brothers," said Jeff Berry, professor of political science at Tufts University. "It was quite a roller coaster of a life."
"Today there are people who survive and come back from their indiscretions," Berry told ABCNews.com. "Time is a gentle father."
"I don't think it damaged him for his whole career," he said. "Gradually, over time, he has grown into the dean of the Democratic Party."
In the past few elections, Kennedy has faced virtually no opposition in his home state and even when Mitt Romney campaigned against him 1994, "nobody had any interest in tarnishing him," said Berry. "Romney knew that would backfire."
The political return of the nation's prodigal son, against the backdrop of today's 24-hour news cycle, opinionated cable television and the blogosphere, may not be that surprising.