It's not hard to explain why this is. In late 1963, Kennedy's approval rating was 58 per cent. Most presidents look better from a distance – even the egregious Jimmy Carter has enjoyed a modest bounce since 1980 –but no president comes close to Kennedy's 27 point gain.

Kennedy's assassination made him, rightly, a tragic figure and embellished a legend that his silver-tongued admirers – many of them former staffers – have burnished ever since.

In reality, Kennedy's achievements in office were limited. He ran on the accusation that Eisenhower had allowed a "missile gap" to develop with the Soviet Union, a gap that Kennedy later found favored the United States. He handled the Cuban Missile Crisis of 1962 well but bungled the Bay of Pigs invasion in 1961. He brought the Peace Corps into being, and signed the Nuclear Test Ban Treaty: both worthy initiatives, but neither transformative. He backed what his successor, Lyndon Johnson, turned into the Civil Rights Act, but could not overcome the opposition to it within his own party. He committed ground forces to South Vietnam: his legacy was a war the US refused to win. He took credit for the Pulitzer-Prize winning Profiles in Courage, which was ghostwritten by his speechwriter, Ted Sorensen.

But what a speechwriter, and what a speaker. Sorensen, who died in late 2010, also drafted Kennedy's inaugural address, in which Kennedy exhorted: "Ask not what your country can do for you: ask what you can do for your country."

The line has been adapted and parodied ever since. In 1972, Nixon offered a counterpoint, urging listeners to "let each of us ask – not just how can government help, but how can I help?" While the sentiment is excellent, the original is better.

But popular conceptions of Nixon and Kennedy are askew. Nixon may have been a Republican, but he went far to create today's big spending, big governing American state. Kennedy, on the other hand, cut the top marginal tax rate from 91 to 70 per cent – yes, Kennedy, a Democrat, cut taxes on "the rich". He believed that growth was more important than socking it to the better-off, and he was rewarded by an economic boom.

Kennedy was no conservative. But he was no feckless liberal either, and his economics, his defence policy, and his foreign policy were all closer to Reagan's than to Nixon's. And it is no coincidence that he and Reagan, the two presidents who most successfully projected optimism, patriotism, and leadership, sit atop Gallup's rankings.

In 1960, Kennedy ran on the charge that Eisenhower had neglected America's allies, and took office with the promise that America would "pay any price, bear any burden, meet any hardship, support any friend, oppose any foe, in order to assure the survival and the success of liberty". The charge was unfair, but the promise was inspirational. While Kennedy spoke as the torch-bearer of "a new generation of Americans," in reality he represented the last gasp of optimistic American liberalism. Only 12 years later, a downbeat Nixon lamented that "at every turn, we have been beset by those who find everything wrong with America and little that is right". Kennedy did less than legend credits him with, but he did not believe he was elected in order to apologise for America.

How times have changed. In his first year in office, President Obama apologised relentlessly, to France, to the "Muslim World' and to Turkey. He tried to "reset" relations with autocratic Russia and dictatorial Iran. Last week, the administration stated that China has received briefings on US nuclear and missile defence policies of the type that "we gave our closest allies". If there is a touchstone of Obama's foreign policy, it is a desire to lead by retreating.

More broadly, the transition from Kennedy to Obama is the story of the decline and fall of American liberalism. In 1963, Martin Luther King Jr. asked that his children be judged not "by the colour of their skin but the content of their character."

Today, liberals plead the case for affirmative action based on race. In 1964, Lyndon Johnson asserted that "the days of the dole in our country are numbered". Last year, Obama went to the mat to create a vast new system of healthcare benefits that, far from abolishing the dole, extended it to everyone.

Obama is separated from Kennedy by more than just 50 years. They are separated by the divide between American liberalism, as embodied by Kennedy, and the New Left of the 1960s, as personified by Obama.

Yet these two presidents do have two things in common.

First, they are both the darlings of the media, who treated Eisenhower and George W Bush as morons, and their successors as visionaries. Time has shown that Ike was wiser, and Kennedy less clever, than he seemed. The same will be true of Bush and Obama.

And second, both come at the end of an era. After Kennedy came the New Left – and, ultimately, Obama. But Obama's vision, like Gordon Brown's, is based on a disdain for foreign policy as a serious endeavour, and a belief that the financial resources of the state are unlimited. Brown has already received his comeuppance. Obama will receive his.

In 50 years, Obama will be seen not a visionary, but as a man who mistook the visions of the post-Kennedy era for reality.

Ted R Bromund is a senior research fellow at The Margaret Thatcher Center for Freedom, at The Heritage Foundation.