Barack Obama will address a crowd of up to 80,000 supporters at a sports stadium in Denver tonight, where he is expected to sharpen his attacks on John McCain even at the risk of being dragged into a fist-fight with his battle-hardened Republican rival.

An elaborate stage — with white columns designed to resemble a classical Greek temple or the White House — has been built for his speech accepting the Democratic presidential nomination.

Critics seized on this as fresh evidence of a self-regard bordering on arrogance — the downfall of many an Ancient Greek hero — that had already been demonstrated by the faux presidential seal used briefly to decorate his podium this summer. Such is the sensitivity of the Obama campaign that it began circulating reports showing that President Bush had also used classical columns at his convention four years ago.

Confetti and fireworks will be shot into the air at the end of Mr Obama’s speech and, while his aides have sought to play down the “rock star theme”, Stevie Wonder and the Oscar-winning actress Jennifer Hudson have been asked to warm up the crowd in Invesco Field stadium.

Mr Obama insists that he does not want to dazzle a television audience, estimated in the tens of millions, with “a bunch of a high-flying rhetoric”. He intends to focus on the kitchen-table economic issues facing voters and contrast himself with a Republican rival who, he says, offers only a continuation of President Bush’s two terms in the White House.

“The single most important thing I have to make clear is the choice we have in November between the same failed policy of the last eight years for the middle class and the new agenda to boost income for Americans and help families who are struggling,” he said in an interview with The Wall Street Journal yesterday.

His speech is also expected to raise, in subtle fashion, the issue of Mr McCain’s age before his 72nd birthday tomorrow. Some lines of attack have already been road-tested, suggesting that on issues such as energy independence, his opponent is trapped in the politics of the past.

Some worried strategists have cautioned that the convention has been too dominated by an effort to tell the life story of Mr Obama and the psycho-drama of brokering unity with the Clintons, when Democrats should be taking the fight to the Republicans.

The convention was the scene for a show of unity that had been painstakingly negotiated over the past few days. A roll call vote of delegates, in which only a few hundred of Hillary Clinton’s supporters were insisting on backing her, was ended with a dramatic flourish by the former First Lady. She arrived on the floor just in time for her own state of New York’s turn, where she proposed her former rival be elected by acclamation.

“With the goal of unity,” she said, “let us declare together in one voice right here, right now, that Barack Obama is our candidate and he will be our president!” This was greeted by several minutes of cheering, dancing in the aisles and chanting.

On Tuesday night Mrs Clinton had paved the way for this moment to steer the party back on course with a gracious, forceful speech that battered Mr McCain and made a direct appeal to those of her supporters still angry at her treatment from the Obama campaign. “I want you to ask yourselves, were you in this campaign just for me? Or were you in it for all the people in this country who feel invisible?” she asked.

A shared agenda with Mr Obama on issues such as universal healthcare and women’s rights meant that her supporters should never contemplate backing the Republicans: “No way. No how. No McCain.” There were some muted grumbles that she had neither spent enough time praising Mr Obama’s personal qualities nor reversed a primary campaign assertion that he was unfit to be commander-in-chief. There was fresh speculation that her push for unity may also have been motivated by a desire to keep herself in the frame for another presidential bid should the Democrats fail to win back the White House.

Attention last night turned towards Bill Clinton, who is significantly less reconciled than his wife to Mr Obama’s victory and remains angry over slights on his record or the way he feels he was “painted as racist during the primary”. Although the former president was expected to offer his version of a unity message with his speech, aides have confirmed that he will not hang around for the address by Mr Obama today.

The former president was distinctly faint in his praise for Mr Obama, saying that Democrats do not “have any choice” but to vote for him. “Hillary made the only argument [that] matters tonight. Nothing else matters. This election is not about a politician,” he said.

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