Despite having feasted on Barack Obama’s soaring oratory and had the Clintons slake their thirst for unity, some Democrats still headed home from Denver yesterday with a familiar anxiety clawing away in their stomachs.

The aura and adulation surrounding their nominee sometimes serves as a distraction from opinion polls that consistently show, despite hostility towards the Republicans, that Mr Obama is not doing as well as he should. He may well get a “bounce” from his speech on Thursday night, but Democrats need little reminder of how Michael Dukakis enjoyed a 17-point lead in 1988 before being buried by a third-term Republican landslide.

Phil Noble, a veteran Democratic strategist and one of Mr Obama’s earliest backers, remains convinced that it will all be OK, saying: “Barack understands timing and temperature better than anyone. He is exceptional and you will see that again in the weeks ahead.”

Democrats, secure in the knowledge that this should be their year, rejected Hillary Clinton for a candidate who offered the excitement of making history while also soothing America’s racial wounds. But the electorate as a whole is different from that in the Democratic primaries: in short, white liberal guilt and an increased African-American turnout does not ensure victory — especially against a Republican opponent in John McCain whose brand is significantly less tarnished than that of his party.

Mr Noble acknowledged that a black man called Barack Hussein Obama running for president was not far off “a woman with green hair and a middle name of Hitler” trying to be British Prime Minister in 1945. This was the context for Mr Obama’s speech on Thursday. He sought to deal, one by one, with the suspicions that clouded the otherwise crystal-clear night at the Invesco Field stadium.

“I get it,” said Mr Obama. “I realise that I am not the likeliest candidate for this office. I don’t fit the typical pedigree, and I haven’t spent my career in the halls of Washington. But I stand before you tonight because all across America something is stirring.”

It was the same spirit, he said, that “45 years ago today brought Americans from every corner of this land to stand together on a Mall in Washington, before Lincoln’s Memorial, and hear a young preacher from Georgia”.

This was a reference to Martin Luther King’s “I have a dream” speech. But Mr Obama did not mention him by name nor address race directly, as he described how people of every race and creed had been told that “in America, our destiny is inextricably linked. That together, our dreams can be one.”

Mr Obama has been plugging a similar message ever since he burst on to the national stage four years ago with a convention speech proclaiming that there was more to bind America together than drive it apart.

He knows many voters suspect that his Harvard education and his liberal friends — just as much as the colour of his skin — means he does not share their values. On Thursday he reached out to them again: “We may not agree on abortion, but surely we can agree on reducing the number of unwanted pregnancies in this country. The reality of gun ownership may be different for hunters in rural Ohio than for those plagued by gang violence in Cleveland, but don’t tell me we can’t uphold the Second Amendment while keeping AK47s out of the hands of criminals.”

For all the fluted beauty of his speech, it was a defensive address rooted in his insecurity at having failed — so far — to close the deal with voters.

Mr Obama first went out of his way to pour praise on Mrs Clinton and her husband, Bill, at the end of a week that had begun with polls showing many of her supporters ready to back Mr McCain. He then sought to link his own exotic family background to the direct experience of ordinary Americans.

Next, acknowledging that some people regarded his language of change as mere “happy talk” from a man high on rhetoric, low on substance, Mr Obama embarked on a detailed tour of his policy programme.

Some observers, used to his eloquence, have suggested that they never sense his anger unless it is directed at those casting aspersions on his patriotism or personal biography. So on Thursday he raised his rich baritone again to attack the squandering of dreams, lives and opportunity under President Bush. “Tonight, I say to the American people, to Democrats and Republicans and Independents across this great land — enough!”

His call to restore a “higher purpose” in America is somewhat undermined by the low-road partisan politics he claims to disdain but still uses to good effect. Pointing out that Mr McCain supported Mr Bush 90 per cent of the time, he said: “I don’t know about you, but I’m not ready to take a 10 per cent chance on change.”

Mr Obama said the Republican, whose 72nd birthday was yesterday, clung to “old, discredited” ideas. But with all the smears and fears that wash around Mr Obama’s candidacy, he could be forgiven if he plays a bit of inter-generational scare politics himself. The “greatest risk” America could take, he said, would be to continue the politics of the past eight years by voting Republican once more. “We need a President who can face the threats of the future, not keep grasping at the ideas of the past,” he said.

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