“Is he lucky?” was supposedly Napoleon's dispositive inquiry about ambitious officers. Barack Obama would surely pass the little general's test.

The financial gale that has blown through the American landscape in the past month has upended the presidential race, ripped the roof off John McCain's Straight Talk Express, propelled Mr Obama gently aloft and landed him, feet first, on the threshold of the White House.

A month ago, emerging from the two party conventions, the 2008 presidential race was beginning to look like another scrappy, down-to-the-wire dogfight, with a slight, developing advantage for the Republican.

With a clever piece of political theatre that simultaneously, through an oddly un-Republican convention, distanced himself from George Bush and - through Sarah Palin - realigned himself with his party's conservative base, Mr McCain was threatening to pull off an improbable victory.

In the final two months, the assumption was that the basic rule of this election would apply with more force than ever - the more the voters' focus was on Mr Obama the more their doubts and questions about him proliferated like a cloud, and the more likely it was that Mr McCain would win.

Strategists on both sides figured that a crisis might upset these calculations, but the probability was that such an event - a terrorist attack or some setback in Iraq or Afghanistan - would actually help the Republican, the traditional intendant of the nation's security.

But then, with a timing that suggests there might really be something divinely ordained about Mr Obama's political rise after all, it was a financial storm that struck.

At its heart was not an evil terrorist or some foreign army, but one of the most demonised anti-heroes of populist iconography, the Wall Street banker, grown fat on free-market excess and Republican tax cuts. Just to make the Democrat's task easier, the self-confessed economic illiterate, Mr McCain, played along helpfully, stumbling drunkenly from uncomprehending gaffe to pointless political stunt to absurdly implausible recovery plan.

Mr Obama had merely to look grave and empathetic, and in the space of two weeks he has opened up a lead in the opinion polls that, if history is any guide, simply can't be overturned in the three and a half weeks that remain to election day.

On the ground, economic hardship, and expectations of much worse, have swung states towards him that have not voted for the Democratic candidate in years. He now not only possesses a solid lead in the crucial swing states that he needs, such as Colorado, Iowa, Ohio and Pennsylvania. He is competitive in states that even successful Democrats have not come close to carrying in the past 40 years: for example Indiana, which has voted Republican in every election since 1968. If the polls hold, he will become the first president since Lyndon Johnson to win more than 50.1 per cent of the vote.

It is not the first time that fickle fortune has beckoned in Mr Obama's direction. In fact, the events of the past month recall almost eerily his very first contest for national prominence in 2004.

When he ran that year for the Senate in Illinois, it was not a perfectly timed financial mess but two exquisitely timed divorces that smoothed the way.

In the Democratic primary, he was a long shot. But a month before the election, his main opponent, Blair Hull, a wealthy Chicago futures trader, was forced to publish divorce papers that revealed, among other charming details, his wife's claim that he had once threatened to kill her.

In the general election, lightning struck again. His opponent, the engaging Jack Ryan, had run a campaign as a different sort of Republican. But a few months before the election, his divorce papers revealed that, while he might have been a different sort of Republican, he was from precisely the same stable of Obama political opponents. He had, it turned out, once tried to force his former wife to go with him to sex clubs in Paris.

As he went on to win that Senate election in a landslide, Mr Obama might have reflected winsomely on the lines from the Bob Dylan song Idiot Wind: “They say I shot a man named Gray and took his wife to Italy,/ She inherited a million bucks and when she died it came to me./ I can't help it if I'm lucky.”

Napoleon wasn't joking when he insisted that his generals be lucky. The Frenchman thought luck was a personal attribute, a quality that distinguished a truly successful general from a merely competent one.

There's much in Mr Obama's record that suggests he has more than dumb luck, but a signal ability to create and exploit it. He may have been the beneficiary of unusual amounts of political sunshine, but he has also been someone who has, as they say, made his weather.

It took political courage and a preternatural self-confidence two years ago to choose to run against the prohibitive favourite, Hillary Clinton, in the Democratic primary. Courage, because in the Democratic Party you cross the Clintons at your peril: it would have been hard to see them rewarding him if Mrs Clinton had won. It took good judgment too: the recognition, when all around him were ready to sign on to the inevitable Clinton machine, that she harboured deep flaws as a candidate.

And for all Mr McCain's fumbling in the past few weeks, it remains the case that the Arizona senator won his party's nomination because, as even some of his opponents acknowledge, he was the one with the best chance of winning the general election. Mr Obama could have had an easier opponent these past few months.

Is there anything now that can stop him riding this unusual wave of time, chance and ability all the way to the White House?

The McCain campaign hopes that, just as happened in the Democratic primary, Mr Obama will stumble as the prize draws near. He lost nine of the last primary contests against Mrs Clinton. The reason, they say, is that, as his selection became inevitable, a sort of premature buyers' remorse set in as voters wondered whether he was really ready to be president.

Perhaps the same will happen in the next three weeks. It seems improbable. The least we can say is this. To win now, Mr McCain would require a stroke of improbable fortune greater than anything that has come Mr Obama's way during his startlingly swift ascent.

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