Barack Obama has come a long way since his first Democratic convention in Los Angeles eight years ago when, by his own account, he was "more or less broke" and still coming to terms with the resounding defeat of his attempt to win a seat in Congress.

At the airport his American Express card was rejected twice and it was only after half an hour of telephone negotiations that he was allowed to hire a car. Later, denied credentials to the convention floor, he spent a desultory few days watching speeches on TV screens and trying to follow friends into "skyboxes where it was clear I didn't belong" before going home early.

Four years later he had a happier experience. Still unknown and not yet elected to the Senate, he burst on to the national stage with a speech in Boston, sending an electric current surging through a party preparing to anoint John Kerry as another wooden, unsuccessful presidential nominee.

If some of those watching him back then thought that maybe - one day - he might run for president, few would have dared to predict that he would be arriving in Denver this week having seen off the meat-grinding Clinton election machine to become the Democrats' choice for president or, as Republicans like to say, "the biggest celebrity in the world".

The inevitable tumult and triumphalism greeting him when he walks out in front of 75,000 people at the mile-high Invesco stadium on Thursday will, however, serve to obscure the great glaring gap in this rags-to-riches story: Mr Obama may not win.

Never mind that this should be a Democratic year with fully four fifths of the voters saying that America is "on the wrong track". Mr Obama is running a long way behind his party. Although polls show that a "generic Democrat" would have double-digit margin over a "generic Republican", Mr Obama's summer lead has evaporated into statistical insignificance.

For all the babble about his "making history" as America's first black president, many voters are more concerned about saving their jobs or paying their mortgages. They cannot afford to make history, nor do they want it thrust upon them by the wealthy coastal elites who idolise Mr Obama.

He won the Democratic nomination with a message of change carried by his own compelling oratory, symbolised by an inter-racial - international - background, propelled by the enthusiasm of young white liberal and African-American voters. But it is easy to forget that he lost the final third of the primary to Hillary Clinton, who succeeded in positioning Mr Obama as an out-of-touch elitist.

Only in the past week have the Democrats recognised that they have a real scrap on their hands. Mr Obama has returned from his holiday in a much sharper, more populist mood, focusing on his promise to turn around the failing economy. It is too late to stop Thursday's event, but Mr Obama insists that he will not be seeking to "dazzle". He wants to dispel the notion that his campaign is "somehow a rock concert", saying that he will be more intent on laying out fundamental choices about economic policy than "a bunch of a high-flying rhetoric".

The strategy will be to reintroduce the Harvard-educated Mr Obama to voters as the product of a "very American story", with family members and ordinary voters paying testimony to his middle-class origins or describing his down-to-earth qualities.

Little is being left to chance in Denver. In an exercise of control freakery more reminiscent of the Beijing Olympics than the language of grassroots empowerment, protesters have been told exactly where and when they can demonstrate. Dozens of metal cages in a warehouse on the outskirts of the city have been set aside for those who misbehave, although police have assured civil rights groups that they have come up "with a way to put a lid on the cells instead of the razor wire" they once planned to use. Homeless people are being offered haircut vouchers and given free cinema tickets.

Mr Obama's gym-honed skinny frame still marks him out from the obesity that ripples across the waistband of America and his convention's host committee has distributed "lean 'n' green" catering guidelines to Denver caterers. These implore them to strike fried food from the menus.

Perhaps Mr Obama should remember that the last time the Democrats held a convention in Denver was 100 years ago when Williams Jennings Bryan, known for his oratory, was chosen to run against the unpopular incumbent Republicans. He lost.