Rupert Cornwell: So is Obama the saviour of his party?

Finally, the warm ups are over – Ted Kennedy and his farewell call to arms, the Clintons and the latest episode of America's favourite political psychodrama. Even the uncertainty about whether the engaging but irredeemably prolix Joe Biden can keep control of the word count will have been resolved when this article appears. Here in Denver, we've at last got around to the one thing that matters. Barack Obama.

Given the breathless build up that precedes them, and the no less breathless analysis that follows them, you might think convention speeches can decide presidential elections. Almost always, however, they are instantly forgotten. But the speech Obama delivers tonight may be the exception. An angst hangs over the gathered representatives of the Democratic party, as seemingly tangible as the Rocky Mountains etched on the city's western horizon. Their foreboding is simple: have we picked the wrong person – and is it possible we are going to lose the most winnable election in a generation.

Every delegate can reel off the advantages that should make their man a certainty to be behind the Oval Office desk come next January: a desperately unpopular Republican President, an unending war, a lousy economy, a gnawing sense of national decline. Even Mother Nature may be about to cooperate, by delivering a major hurricane in the Gulf of Mexico in the midst of the Republicans' own convention next week, almost three years to the day after Katrina. No wonder in the "generic" polls that measure broad party preference, Democrats have a huge lead, of 12 per cent or more.

But when it comes to specifics, i.e. Barack Obama versus John McCain, their man is running only level. Now McCain might be the best Republican option in this inauspicious year, but he's not exactly a superb campaigner. He's old, a dismal public speaker, and distinctly gaffe-prone. The reason he is holding his own is that people simply don't yet have confidence his untested young opponent is up to the job. Tonight is an opportunity to overcome those doubts, in a prime time speech in which the candidate will have the country's undivided attention. And rightly so. For not only this convention, but this entire election, is now about one thing only: Barack Obama.

Even the Clintons have been a sideshow. Hillary gave a magnificent speech, all that Obama could have asked for and more. Yes, rumours persist about devious Clinton manoeuvres to bring about an Obama loss in 2008, setting her up for a second run in 2012. It has been noted too that, however forthright her support, Hillary did not explicitly state that her conqueror was qualified to take that famous 3am call in the White House. Bill for his part may skip tonight's grand finale.

But ultimately, none of this matters. The nominee's speech will stand or fall on its own merits, whether or not Bill is there to watch. And it is not up to Hillary to stipulate whether her former rival is fit to be commander in chief. It's up to Obama. Nor does Hillary's endorsement alone guarantee that her diehard supporters will return to the fold. Obama must win them back. And he must do it not with symbols, but with specifics.

If anything, in the case of Barack Obama, the country is suffering from symbolism overload. Everyone knows the candidate's astonishing life story, and all that it represents. The very date of tonight's address, the 45th anniversary of Martin Luther King's I Have a Dream speech, is yet another symbol.

In retrospect, the Obama campaign early on was less a candidacy than a movement, inspired by broad themes and soaring oratory. Much of the angst here stems from the fear that it peaked too early. By the end, it was Hillary, not he, who was on a roll. Inspirational mass rallies were out, and facts – which she seemed to supply more of – were in. Moreover, since the end of the primary season the candidate has seemed curiously detached and devoid of passion. If anything, external events have moved even further in the Democrats' favour. But Obama himself has yet to articulate a simple, specific, message.

Part of the trouble is the style of the man. The American political system has an inherent, and sometimes very expensive weakness – that the qualities needed while campaigning for President tend to be the those least suited to being President. During campaigns everything is presented in stark black and white, and George Bush the "decider" is proof of where an unadorned good versus evil approach to the world can get you.

One of the most unusual and attractive features of candidate Obama is that he visibly thinks before answering a question, weighing the pros and cons of an argument. That is an excellent quality for a President. But on the campaign trail it can come across as a lack of urgency. Obama moreover faces an additional difficulty. All along, his candidacy was presented as non-confrontational, based on the premise that he could bring Americans together. Stop thinking and start fighting, worried Democrats now urge. For some, uncomfortable parallels come to mind. Jimmy Carter once ran a campaign as clinically efficient as that of Obama, but probably was too nice to succeed as President.

But Democrats should not despair. Maybe, in those heady days of January and February, Obama wasn't quite the miracle worker he appeared. But now, by the same token, he is probably a better candidate than he has seemed over the last few weeks, as McCain has caught him up in the polls. Angst obscures another truth, that Obama is a quite remarkable politician. He has not only shown a relentless single mindedness to win the crown. As his speech on race after the Jeremiah Wright controversy proved, he can take on the very biggest issues, as few others can.

Of course race, the great unknown of this campaign, could yet be his undoing. But that is an issue beyond Obama's power. What he must do now is to make the sale. He must convince Americans that he is not merely a philosopher, but someone who grasps the problems of everyday life, of stagnant wages, vanishing jobs and foreclosed homes. Symbols cannot do this, vice-president Joe Biden cannot do this, even Hillary Clinton cannot do this. Only Barack Obama can. But if he can make the sale, then a neck and neck contest may turn into the sweeping victory of which those angst-ridden delegates in Denver dream.

For rolling comment on the US election visit: independent.co.uk/campaign08