Democrats are here for a coronation, but at times this convention will be something like a seance.

John F. Kennedy, had he lived, would now be 87. In the four decades since his death, Democratic conventions have always had a moment when the party stops to pay homage to a martyred president. Never, though, has there been quite the same convergence -- a harmonic one, Democrats hope -- of political past and present as here in Boston, with the JFK legacy hovering over the proceedings.

It will be explicit Tuesday evening, when Sen. Edward M. Kennedy -- who, in the autumn of his own career, lobbied hard to bring the convention to his and his brother's home town -- addresses Democrats at FleetCenter. It is likely to be more understated on Thursday night, when John F. Kerry, Massachusetts's junior senator, delivers an acceptance speech for his party's presidential nomination. This will be the high point so far in a political life powerfully influenced by the Kennedy example. As a young man, Kerry even faced criticism that he was laying on his emulation -- the accent, the hair, the initials, the attempt to infuse politics with an aura of dashing glamour and celebrity -- a bit too thick.

Three generations of Kennedys have gone through many seasons of turmoil and renewal in the 44 years since the 16-year-old Kerry experienced a political crush. Concern with how the legacy resonates -- whether it evokes images of public service or private scandal, timeless idealism or an antiquated liberalism -- was one reason some Democrats were wary about coming to Boston.

The rush of Kennedy reminiscence underscores the ambiguous influence the 35th president has had on the politicians who came in his wake. His luminescent image and idealistic calls to service inspired a generation of ambitious Democratic candidates -- but in a variety of political and even psychological ways have also been a burden.

Despite the controversies that have swirled since Kennedy's death -- well-documented accounts of adultery, public deception over the state of his health and continuing debate over whether he led the country astray in Vietnam -- surveys routinely place him at or near the top of 20th century presidents in public esteem, noted historian Robert Dallek, the author of a recent Kennedy biography.

Because of the mythology that grew after Kennedy's death, he said, the question for any current Democrat is, "How do you measure up? It's not something easy to live up to."

In Kerry's case, some people believe that his speaking style has been hindered by deliberate or subconscious efforts to echo the sonorous language of the New Frontier. Such rhetoric, typified by the elegant cadences of the 1961 inaugural address, can sound bloated and pretentious to modern ears.

"Elevated prose can really distance a speaker from his audience," said Jeff Shesol, a former speechwriter for Bill Clinton who has written a book on Robert F. Kennedy's 1960s feud with President Lyndon B. Johnson. While declining to critique Kerry specifically, he said politicians who try to match Kennedy's rhetorical style can "sound like a parody."

The Kennedy influence over the years has been problematic for Democrats in other ways. His merger of politics and sex appeal -- and, it later became clear, risk-taking and sexual adventurism -- was a dangerous example for politicians after him, given changing public attitudes and a political and news media culture that is less protective of the private lives of public figures. By coincidence or not, several Democrats over the past several decades who self-consciously identified themselves with Kennedy have come to grief.

Former Colorado senator Gary Hart, who was drawn to political life by the Kennedy example and whose public style borrowed from him, was forced to drop out of the presidential race in 1988 after a sex scandal. Even earlier, Ted Kennedy himself was badly wounded by questions over his personal conduct after a car crash in which a woman riding in his car drowned. Clinton's career survived his scandal involving a White House intern, but at considerable cost to his presidency.

Dick Morris, his longtime consultant, said Clinton was intrigued by Kennedy's private life, and may have concluded that "sexual perks came as a side benefit to a life of public service and sacrifice." In his White House memoir, Morris wrote that Clinton once said that if Kennedy came back to life he wished he could ask him, "you know, how'd you do it, how'd you get away with it?"

Kerry's career has never been grazed by this type of personal controversy. On the other hand, it is unlikely that he -- or any modern politician -- could electrify a generation the way Kerry and other young Democrats were awakened by Kennedy.

"Kennedy just captured our attention and imagination," said John Shattuck, who attended St. Paul's School with Kerry, where they were among the few Kennedy enthusiasts on the New Hampshire prep school campus. "There was an almost instant bond with the idea of youth and vigor and public service."

A dozen years ago, when Clinton was nominated for the first time, a video of him as a 16-year-old Arkansan shaking hands with Kennedy in the Rose Garden during a Boys Nation field trip sent chills up the spine of the convention hall. It seemed somehow to be destiny.

Kerry's encounter with Kennedy was more intimate. In 1962, he dated Janet Auchincloss, the half-sister of Jacqueline Kennedy. One summer day, he was invited to her family's Rhode Island oceanside estate, and discovered to his delight that the president was there. A photograph of Kerry on a sailboat with the president has become as much a part of his biography as the picture of Clinton with Kennedy. Kerry has frequently described the sense of despair and lost possibility he felt after Kennedy was assassinated the next year.

Dallek said it was not just an unexpected death, but a sense that the future had been robbed that gave Kennedy's loss such a long echo. The most recent president to be assassinated before that was William McKinley in 1901, a death that also occasioned an outpouring of national grief at the time, but was hardly remembered 40 years later.

Still, it is clear the years have scuffed the Kennedy name.

Among many voters -- the youngest eligible to cast ballots this year were born in 1986 -- it is not clear the late president evokes any emotional connection.

Sen. Joseph R. Biden Jr. (D-Del.) said he wishes the current generation could feel what he experienced when Kennedy came to power. "It meant unlimited possibility. It meant the idea that public service was a noble mission," he said. He said he believes Kerry could summon a similar spirit of national sacrifice to meet the challenges of an era shadowed by terrorism.

Shattuck, who remains a Kerry friend and is the chief executive of the John F. Kennedy Library Foundation, said comparisons between circumstances and personalities from different eras can go only so far. "My sense is [Kerry] is very different from Kennedy as a matter of personal style," he said. "There may be some parallels, but I don't think we're trying to reelect Kennedy."

Maria Shriver attends the dedication Sunday of the Rose Fitzgerald Kennedy Greenway with her parents, Sargent Shriver and Eunice Kennedy Shriver.Robert F. Kennedy Jr. attends the dedication to his grandmother in Boston, the Kennedy family's home town, with sons Aiden, 6, and Fenbar, 3.President John F. Kennedy's image and calls to service inspired Democratic candidates.