This race “is the most important election of my lifetime”, Kennedy has said. He added that he has not worked as hard in a presidential election since his own bid for the White House in 1980.

Kerry says that he talks to Kennedy often and owes him more than he can say. He has accepted the patronage with enthusiasm and gratitude, far more than his friends expected — and more than they might have advised.

On balance Kennedy’s stature and his vigorous efforts have helped Kerry more than they have hurt him. But for Kerry, close association with so controversial and charismatic a politician has carried a risk.

Kennedy was born into the Democratic Party’s greatest dynasty. He became a senator for Massachusetts in 1962, when he reached 30, the youngest age permitted by the Constitution. After the assassination of his two brothers, it was inevitable that many saw him as the heir apparent, all but certain to move smoothly into the White House.

But his career has been overshadowed by one word: Chappaquiddick. In 1969, driving on a small island near Martha’s Vineyard, his car left a low-sided, curving bridge. His companion, Mary Jo Kopechne, drowned; worse, Kennedy’s account of his attempted rescue was evasive and inconsistent.

More than a decade later the sense of political blight remained. In the 1980 presidential race he was roundly defeated at the primary stage by Jimmy Carter, the sitting President.

So he stayed in the Senate, where he has become one of only five senators in history to last more than 40 years; he says he will run again in 2006. He is an elder statesman of Democratic liberalism: one of the first and fiercest cham-pions of civil rights.

In Massachusetts he is a legend, adored for his record on the state’s behalf. He has secured billions of dollars for the Big Dig, a vast project to reroute Boston’s flyovers underground, which is hugely over budget, but has come to symbolise the city’s renaissance. At past Democratic conventions he has delivered some of the party’s greatest rhetorical moments, including “The Dream Shall Never Die” in 1980 and “Where was George?” in 1988.

Kennedy and Kerry first met in 1971, when Kerry came to Washington, organising a protest of Vietnam veterans against the war. A wary friendship began, fired by Kerry’s childhood interest in the Kennedy family.

As characters they are opposites. Kerry is aloof, even shy. Now 60, he manages a heavy youthfulness with bursts of showy athleticism. Kennedy, 72, expansive in temperament and physique, looks a generation older: white-haired, red-faced and bulky, the attributes of senior-ity in a bygone political age.

As the senior senator (the first of the Massachusetts pair to be elected) Kennedy was in a position to help Kerry — and to overshadow him. He has done both.

Kennedy endorsed Kerry in January last year, when many candidates had not even decided to run. He has since toured the country to raise money for him. Late last year, when the Kerry campaign seemed to have died, Kennedy urged that Kerry hire his chief of staff. Many date the campaign’s rebirth from that point. In the past month Kennedy fixed — almost — the row between Boston and the police unions that threatened to drag down the convention, and he extracted $25 million (£13.5 million) in federal cash to pay for the huge security presence.

But he has also taken on Bush with a clarity and passion that Kerry has never mustered, from an aggressive, liberal position that Kerry might well feel it would be dangerous to share.

He has mauled Bush over healthcare for the elderly and education for poor children, where Kerry has been muted, nervous of being tarred as a tax-and-spend liberal.

He has attacked Bush over Iraq more than Kerry has felt it prudent to do, calling the war one of the worst blunders in the history of US foreign policy.

Kennedy’s own desire to take centre stage is oozing out of this week’s schedule: the “Tribute to Ted” concert at Symphony Hall; his dedication of a public green space in honour of his mother, Rose Kennedy; and a celebrity gathering at the Robert F. Kennedy memorial. He is filling a hundred hotel rooms with his family and friends, according to reports.

The question tonight is whether he can deploy his rhetoric, stature and command of the stage in his own town in Kerry’s best interests, and help his protégé to fulfil the dream denied to him, by reaching the White House.

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