Sixteen years ago, as the worst race riots in modern American history engulfed Los Angeles, a young black man stood blinking before a phalanx of television cameras and asked: “Can we all get along?”

Rodney King's question was for his city, but more especially for his country. In that year, the United States saw ugly proof on every rolling news channel that its social fabric was still riven along racial lines despite the great sermons and legislative triumphs of the civil rights era; despite the subtler preaching of The Cosby Show and The Fresh Prince of Bel Air; despite decades of affirmative action in its armed forces and universities; and despite the yearnings and convictions of the most shamelessly optimistic electorate on Earth.

Since then, the failed presidential campaigns of Jesse Jackson and Al Sharpton have dented some of that optimism. They forced voters to ask not only whether Americans could ever elect a black president, but whether a black candidate could ever sublimate his country's tortured racial history and move beyond it, to offer a message of colour-blind progress rather than confrontation.

Such questions have been answered by Barack Obama in a way that has already rekindled America's faith in its prodigious powers of reinvention - and the world's admiration for America. He could still lose the White House to John McCain. It has been a bruising journey from the Iowa caucuses to Minneapolis, where he staked his claim last night to the Democratic nomination. But today at least the tide of history seems to be with him. Win or lose in November, he will have gone farther than anyone in history to bury the toxic enmity that fuelled America's civil war and has haunted it ever since.

It is worth rehearsing Mr Obama's emergence into the media spotlight two years ago, not so much for what it says about his undoubted political talents as for what it reveals about the US political system. Early in President Bush's second term it was already clear, even to many of his supporters, that he had gambled boldly with his country's prestige and self-belief, and lost. No whistle was required to start the race to find someone to rebuild those crucial components of democratic strength. By 2005 both main parties were canvassing dozens of potential candidates, noting experience and name recognition, but seeking more. In 2006, the national media started paying attention to the eloquent son of an African goatherd with a fervent following among Democrats in Illinois. His campaign to derail Hillary Clinton and become the first black president has since electrified the world.

Details of the delegate count no longer matter. This moment's significance is its resounding proof of the triusm about America as a land of opportunity: Mr Obama's opportunity to graduate from Harvard and take Washington by storm; the opportunity that the world's most responsive democratic system gives its voters to be inspired by an unknown; the opportunity that outsiders now have to reassess the superpower that too many of them love to hate.

For a generation, the politics of America has been commodified by pollsters and analysts. Its political landscape has been minutely mapped; its new online constituencies targeted by “dog whistles” and YouTube narrowcasts. Mr Obama has torn up much of these analysts' conventional wisdom with what he calls the audacity of hope. For what? His promises of unity and change are vague. His critics say that the ranting of his former pastor shows them also to be empty. But he has survived such claims, and may be tougher for it. His Republican opponent, “too tough to die”, embodies many strengths that Mr Obama can only applaud. But he has his own. The epic continues. Act II starts now.