There's a fundamental paradox confronting Barack Obama as he prepares to deliver his acceptance speech to the Democratic Convention before a stadium crowd of up to 80,000 tonight.

Speech-making is what Senator Obama does best. It is arguably the main reason that he finds himself on the threshold of the White House. He is one of the finest exponents of English oratory our American cousins have produced; right up there with Martin Luther King, William Jennings Bryan and Abraham Lincoln.

And so, barring some unimaginable collapse by either his speechwriters or the candidate himself tonight, he will give a barn-burning performance that will send his anxious Democrats away energised and girded for the battle that unfolds in the next two months. But here's the paradox. Senator Obama's biggest challenge from now until election day is to prove that he is more than just a great orator. His very ability to move audiences with words and dramatic set-piece performances is now part of the principal critique of him by his opponents: that he is a profoundly inexperienced young man who has done no more in his short public life than give great speeches. The more he inspires and moves his followers with great speeches the more glaring the gap between his speaking talent and his callowness appears.

There are growing indications, in fact, that it is the now familiar Grand Obama Occasion of the sort we will see tonight that is starting to turn voters off him. It may bring the people in the stadium to tears but its impact on the wider American audience watching on television is much less certain.

His current troubles really seemed to begin when he gave his famous speech last month to 200,000 Germans in Berlin. Europeans loved it and embraced its message. But back in the US it looked oddly empty. It seemed hubristic for this man who has achieved nothing of substance in his life - all promise and no record - to receive the adulation of an entire continent.

So tonight poses tremendous pitfalls for him. The spectacle of another vast crowd gathered in an open-air stadium under the dramatic and all-American Rocky Mountain Big Sky might just again look a little too much.

His advisers supposedly toyed with the idea of having him accompanied tonight by a performance from Bruce Springsteen, but wisely decided not to. One of the most effective attacks by the John McCain campaign in the past month has been the ridiculing of him as a celebrity in the mould of Britney Spears. If he had shown up before a vast crowd in the company of The Boss it would only have reinforced that message.

The white-columned set being built for Senator Obama is intended to mimic the marble halls of Washington; though there's even danger of hubris in this.

It seems to evoke the columns of the Lincoln Memorial in Washington, where 45 years ago to the day Martin Luther King delivered one of the most famous speeches in American history. Direct self-reference to the greatest civil rights leader in American history might also seem over the top to some viewers.

Tougher still is the content of the speech. It's axiomatic now that Senator Obama's biggest electoral problem is his failure to connect his soaring rhetoric of change and hope with the anxieties of ordinary voters struggling in a stagnant economy. He is urged repeatedly by pollsters to be more specific and directly empathetic. Yet he cannot allow his speech tonight to be a laundry list of tax credits and regulatory proposals. Instead, he must find verbal ways of demonstrating that he feels Americans' pain, as Bill Clinton once famously put it and that, despite the image of a lofty elitist, he is in touch with voters' everyday concerns.

It's routinely said of a particular speech by a politician that it is the most important of his life. But for a candidacy built on the unsteady scaffolding of oratory, in a strange way the opposite is true. Everyone knows he can speak. Can he govern?

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