“Both Kennedy and Obama have fantastically winning smiles and I might say both are very relaxed in front of an audience and on television," he said in an interview. "They don’t shout into a microphone, they talk.

"The principles, the values Obama and Kennedy are enunciating are not five-point plans for new health care programmes, which is more Hillary's style."

The Kennedy legacy and the aura of Camelot have been powerful but largely unspoken themes underpinning the campaign of Mr Obama, another charismatic Harvard alumnus heralding a new era in politics.

Mr Kennedy broke down the barrier of becoming the first Roman Catholic president while Mr Obama is vying to become the first black occupant of the Oval Office.

Just as the Massachusetts senator was dismissed by the party hierarchy because of his age and unconventional background, Mr Obama, too, is being branded an also-ran.

Now, the endorsement of Mr Sorensen, who worked for Mr Kennedy for 11 years and wrote the 1961 inaugural speech in which he spoke of a torch having been "passed to a new generation", has made the linkage explicit and given it official sanction. More pointedly, the Democratic veteran expressed what few senior party figures are prepared to do publicly — the conviction that Mrs Clinton will lose a general election and in practice is not much different from President George W Bush. When asked about her similarities to Bill Clinton, Mr Sorensen said that her election would be "a continuation of the Clinton-Bush 20 years" and business as usual in Washington.

“She has the same tendency to triangulate, as he called it, she has the same ability to equivocate, to vacillate, to imitate what the Republicans are doing and saying.”

He vehemently rejected the contention, driven home relentlessly by the Clinton campaign, that Mr Obama lacks the experience to be president. "He has great judgment, which he has demonstrated in his position against the Iraq war even before it started. "Judgment is the single most important criterion for selecting a president.

 At the time of the [1962] Cuban missile crisis, Kennedy's powers of judgment were tested as no president has ever been tested. Fortunately for all of us, he really came up with the right answers. He was 45. Obama's 46 so he's an old geezer."

Mr Sorensen's eyes are now failing him but when he is at an Obama rally the message, mannerisms and atmosphere make it seem like the 1960 campaign once again.

"I've heard a lot of high pitched shrieks of approval, which I assume were coming from young women," he said. "I'm told the phenomenon known as leapers has returned. "In Kennedy's case, along the motorcade route young women would levitate themselves to be able to see over the heads of taller people as he was driven by.

"There was excitement and enthusiasm that went into that leap as well as his good looks. It's interesting they report leaper sightings when Obama appears." But perhaps the thing that most makes Mr Obama, the first-term Illinois senator, "on track" to become the new Mr Kennedy is his determination to transform American politics.

"There's a sense in this country that Washington badly needs to be changed. The election of Obama will not only change the players in Washington, it'll change the game itself."

The Clintons, he said, were Washington insiders who wanted to maintain the status quo and who had not brought honour to the White House. “I’m not accusing Clinton of being lawless. He was impeached for trivial reasons. "But I don’t think that it was the noblest time for the White House when the Lincoln bedroom was rented out to donors and pardons were being issued to some truly dreadful people.” Mr Clinton, moreover, had failed to deliver. "Bill Clinton was a persuasive communicator. If he was here now he'd persuade us both that I'm wrong. But what was it used for? It was sad to see a president with such talent who was unable to get more done."

He argued that the former First Lady would be defeated in a presidential fight with a Republican candidate like Rudy Giuliani. "She's got everything going for her but a lot of people just don't like her. "I'm tired of losing. We've had these candidates who give those five-point programmes, who sound like they are lecturing at MIT or trying to convince the New York Times board of editors. "That doesn't reach the hearts of the voters. Mr Kennedy reached the hearts of voters. And so does Obama."