Every decade or so the people who control the way we see the world anoint some American politician the Redeemer of a Troubled Planet.

In the late 1960s the media placed the halo on Robert Kennedy, the tragic dynast whose antiwar and civil rights credentials made him in life - as he remains to this day in death - a kind of devotional figure for most political journalists.

Kennedy at least had charisma and intelligence. But to prove that these were by no means necessary preconditions for the honour, it was conferred a few years later on Jimmy Carter, the plodding nonentity elevated by a willingly compliant press into Everyman, brandishing his steely sword of Truth against the Manichean mendacity of Richard Nixon's Republican legacy.

Partly because of the Carter embarrassment, the 1980s were barren years for the idolators. Try as they might, they couldn't work themselves into much ecstasy over Walter Mondale in 1984 or Michael Dukakis in 1988, though they had little flings with bit-part players Gary Hart and (I kid you not) Bruce Babbitt, a genial former Governor of Arizona.

But by the 1990s a new Democrat, or rather a New Democrat, was come among us, a man the media told us would lift our eyes from our selfish greed and rid the world of the ineffable misery left by 12 years of reactionary rule. It's hard to imagine now, after the battering he's taken from his old friends in the press these past few months, but Bill Clinton was once their idol. His cleverly cynical balancing act - promising a return to high-minded tolerance while executing mentally ill prisoners in Arkansas, for example - was lauded as a brilliant synthesising of traditional liberal ideology with the political realities of the modern age.

The alert among you will have noticed by now that what all these spiritually uplifting leaders have in common. They are all Democrats. Never in any of the chapters of this hagiography does a Republican, a conservative, appear in a remotely similar light. These alien creatures by contrast have always been portrayed as cartoonish representatives of the Dark Side of humanity, or, if they were really lucky, simply idiots, failed B-movie actors and irredeemably ignorant hicks with embarrassingly neanderthal views on women, religion and communism.

It's been a while coming - neither Al Gore in 2000 (before the luminescence created by his recent joint Nobel/Oscar triumphs) nor John Kerry in 2004 quite fit the bill. But it's fairly clear now that, with the near-certain nomination by the Democrats of Barack Obama everything is in place for the media to indulge in one of the greatest, orgiastic media fiestas of hero-worship since Elvis Presley.

You will not see a finer example of the genre than the cover story of this week's Newsweek, which was entitled “The O Team”. This rhapsodic inside account of Senator Obama's campaign reads a little like a cross between Father Alban Butler's Life of St Francis and the sort of authorised biography of Kim Jong Il you can pick up in any good bookshop in Pyongyang.

Mr Obama is portrayed throughout as an immanently benevolent figure. Not human really, more a comforting presence, a light source. He is always eager to listen to all aides of an argument, always instilling confidence in the weak-willed, resolutely sticking to his high principles and tirelessly spurning the low road of electoral politics. I stopped reading after a while but I'm sure by the end he was healing the sick, comforting the dying, restoring sight to the blind and setting prisoners free.

The panegyric included the now conventional wisdom in the media that Republicans have only ever won elections in the past 40 years through lies and fearmongering - smearing their opponents and spreading false fears that a vote for a Democrat would open the country to foreign invasion.

To be fair, the Newsweek credo was only the latest and perhaps most shameless phase of the pro-Obama liturgy in the media. Some cable TV channels prostrate themselves nightly before him. Most newspapers worship at the altar. They have already set up a neat narrative for the election between Senator Obama and John McCain in November - the Second Coming versus Old Grouchy, The Little Flower of Illinois up against the Scaremongering Axeman from Arizona.

There's a special irony here. Senator McCain is the Republican who has received probably the single most favourable treatment from the media in the past 40 years. He has been a favourite because he conformed to the first law of contemporary political journalism: the only good conservative is a bad conservative. His willingness to defy his party on everything from taxes to global warming, to take on George Bush, has earned him at least an honourable mention in the martyrology of American politics of the past 40 years.

But now that he's up against Oh! Bama! he will have to be recast in the more familiar Republican mould of villain and scaremonger-in-chief.

This media narrative is not only an outgrowth of the journalists' natural enthusiasm for a Democrat such as Mr Obama. It is also a clever ploy to pre-emptively de-legitimise any Republican critique of the Democratic nominee. It is designed to prevent Mr McCain from asking reasonable questions about Mr Obama's strikingly vacuous political background, or raising doubts about his credentials for the presidency.

The idolatry of Mr Obama is a shame, really. The Illinois senator is indeed, an unusually talented, inspiring and charismatic figure. His very ethnicity offers an exciting departure. But he is not a saint. He is a smart and eloquent man with a personal history that is startlingly shallow set against the scale of the office he seeks to hold. It is not only legitimate, but necessary, to scrutinise his past and infer what it might tell us about his beliefs, in the absence of the normal record of achievement expected in a presidential nominee.

If the past 40 years have taught us anything they have surely taught that premature canonisation is an almost certain guarantee of subsequent deep disappointment.

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