No US presidency is as hard to assess as that of John F Kennedy. Fifty years have passed since that shocking day in Dallas, yet for an entire generation of Americans his assassination remains the most traumatic news event of their lives, exceeding even 9/11. Over five decades the myth has only solidified: of a handsome, talented young leader cut down, an era of boundless promise cut brutally short.

In fact, by most measures, Mr Kennedy’s political legacy is modest. He was not a bold liberal reformer as legend has it; rather, he was a cautious and pragmatic moderate. While he appeared to be hitting his stride in his final year in office, it was his successor, Lyndon Johnson, who enacted historic welfare and civil rights reform. Whether Mr Kennedy would have kept the US out of full-scale war in Vietnam – as his supporters have maintained – can never be known. And yet his presidency and premature death mark a watershed in US history with a melancholy relevance today.

During the Kennedy years government inspired respect rather than the corrosive cynicism that politics attracts today, on both sides of the Atlantic. America was still enjoying the 1950s economic boom that created a new middle class. What could be more fitting than a glamorous new President and his family, to imprint this spreading prosperity with elegance and style? Mr Kennedy was forgiven much that would never be forgiven now. His serial philandering was ignored. So, more importantly, was his frail health. His ailments, for instance, contributed to the dismal showing at the 1961 Vienna summit that emboldened the Soviet Union’s Nikita Khrushchev to provoke the Cuban missile crisis just the following year.

In many respects, not least the role of the media, this was the first modern presidency. No previous occupant of the White House instinctively understood the media so well, to the benefit of both sides. The Kennedy assassination, moreover, was when television came of age, when people turned to it, rather than radio or newspapers, for coverage of the news.

The three major networks of those days have been replaced by today’s raucous, round-the-clock cable TV and internet universe, whose insatiable appetite and relentless opinionising have helped fuel the hyper-partisanship that has made Washington politics so dysfunctional. In that sense, JFK encouraged the beast that has hounded his successors.

Inherent to legends is the hope they can be reborn. Barack Obama was hailed as a “new Kennedy” when he became President – not least because the surviving family conferred its blessing upon him during the 2008 Democratic primaries. Consider Mr Obama now, after almost five years in the Oval Office. His administration has been remarkably free of scandal; there is no hint of womanising, or of health problems concealed. Yet his presidency seems to shrivel by the day. One reason is the disastrous debut of his signature healthcare reform. The darker canvas, however, is the struggle of the middle class that boomed in the Kennedy years to even maintain its standard of living, let alone improve it.

Would JFK have coped better than Mr Obama? No one can say. He died too early to disappoint, leaving only a legend.