Forty years on, America mourns and asks: what if Kennedy had lived?

What if? Once again, on the haunted date of 22 November, the question resonated quietly around America and the world. What course might history have taken if President John Fitzgerald Kennedy had not been assassinated in Dallas exactly 40 years before?

In the capital he briefly ruled and in the city where he died, commemorations were low key yesterday. In Washington, DC, his brother Edward Kennedy, daughter Caroline Kennedy Schlossberg and other family members laid flowers on his grave at Arlington National Cemetery.

In Dallas, police closed off the strip of Elm Street running through Dealey Plaza, with its white-painted "X" in the middle lane marking the spot where the fatal bullet struck.

By evening thousands of people had visited the site, some of them leaving small wreaths and markers on the grass alongside.

Had he lived, the leader, frozen by death into eternal youth, would today be 86. Lee Harvey Oswald, the small and resentful man who killed him, would be 64 - had he not been gunned down in turn by Jack Ruby in the act that has sealed a thousand conspiracy theories.

Four decades on, JFK still fascinates America. But the focus has subtly changed. Yes, 75 per cent of the country remains convinced that Oswald did not act alone, and another revelation of presidential philandering can still generate a fleeting media thrill. But in this era of George Bush and rampant anti-Americanism, the Kennedy era stands in new relief. As Ted Sorenson, Kennedy's former close aide and speech-writer, said last week: "In JFK's day, people admired the United States not because of him, but because of the values of the US, not its military might, not its wealth. But that's not quite true of America these days.

"This country's role has suddenly changed from being the leader of freedom to being the country that often acts like a schoolyard bully, insulting our old allies and turning our back on the United Nations."

He added: "The world was totally changed by November 1963."

So what might have happened, had Oswald never succeeded? There is every probability that Kennedy - then at precisely the same point in his presidency as Mr Bush is today - would have been comfortably re-elected in November 1964. His approval rating in autumn 1963 was almost 60 per cent - higher than Mr Bush's now - the economy was thriving and Vietnam caused only minor anxiety.

The critical question is whether Kennedy would have escalated the US presence in Vietnam, as his successor Lyndon Johnson did with such fatal consequences. Historians are divided on the issue, but let's assume for a moment that Kennedy had started to pull the troops out.

A Democrat, rather than Richard Nixon, would probably have won in 1968. Thus no Watergate, and no President Jimmy Carter. Ronald Reagan might have come to power, but as Sean Wilentz argued in Friday's New York Times, there might have been no Republican "Southern Strategy" along the lines conceived by Nixon, which has produced the most important shift in the US electoral map of the past half-century.

Bushes have now replaced Kennedys as America's most successful political dynasty. But the similarities between the President of 1963 and the one of 2003 are striking. Measured by the popular vote, Kennedy's win in 1960 was even tighter than Al Gore's victory in vain in 2000: just 112,000 votes (0.1 per cent of the vote) compared with the Gore margin of 537,000 votes, or 0.4 per cent.

Both have especially loyal and ruthless henchmen: in JFK's case his brother Robert; for this Bush the likes of his political strategist Karl Rove and his Vice-President, Dick Cheney. And now, as then, their supporters may have urged America into mistaken wars. Then it was "the best and the brightest"; now it is the neo-cons who have Mr Bush's ear over Iraq. As the author David Halberstam wrote of Kennedy's and Johnson's advisers over Vietnam: "For all their brilliance and sense of themselves, they had been unwilling to look and learn from the past. They were swept forward ... by the sense of power and glory, of the omnipotence and omniscience of America."

Sound familiar?