In Dreams From My Father, Barack Obama’s highly-reviewed memoirs of his early years, he wrote of a moment in the early 1980s after he took a job on Wall Street when he faced a crossroads about where to go in life. A few months later he took on the radically different role of a community activist among Chicago’s urban poor. He would go on to gain admission to Harvard as a high-flying law student, but that was still several years ahead.

“Sometimes, coming out of an interview with Japanese financiers or German bond traders, I would catch my reflection in the elevator doors – in a suit and tie, a briefcase in my hand – and for a split second I would imagine myself as a captain of industry, barking out orders, closing the deal, before I remembered who it was that I had told myself I wanted to be.”

More than any other candidate in either the Democratic or the Republican field, Barack Obama has woven the key policy messages of his campaign into a story about his personal history. Much as Bill Clinton in 1992 married his promise of economic opportunity with the biography of a poor kid who grew up “in a town called Hope”, Mr Obama has fused his personal and political messages into a coherent campaign narrative.

In contrast to Hillary Clinton, whose relatively mainstream “middle class, middle American” upbringing only tangentially connects with her central campaign promise of bringing experience and competence to the White House, Mr Obama’s core message is delivered in one piece – the man and his platform are merged. It is a very American narrative – with undertones of Christian redemption – about emerging from humble beginnings and a mixed racial background to make good in the world.

Born in Hawaii to a Kenyan father (whom he barely knew) and to a white mother from Kansas, Mr Obama has been criticised by more conventional African-Americans, including Jesse Jackson, for “acting too white”. But Mr Obama has succeeded in converting his mixed ethnic background into a novel persona in which he can remain black while appealing to whites without – in the words of one commentator – “reminding them the whole time that they are white [something Mr Jackson allegedly does]”.

In the same way that Mr Obama pitches his appeal at both “red state and blue state America” – Republican and Democratic – he is also winning support in overwhelmingly white states, such as Iowa, which is 96 per cent caucasian and which, until Thursday night, had never voted for an African-American, as well as in states such as South Carolina, which is 30 per cent black and where he is now leading in the polls for its primary election later this month.

When Mr Obama invites other African-Americans to join him on the campaign stump he prefers figures with multi-racial appeal, such as Oprah Winfrey, whose television show reaches mass audiences, and Deval Patrick, the first black governor of Massachusetts, over traditional black politicians such as Mr Jackson or Al Sharpton.

“Barack offers African-Americans the opportunity to get their first black president and he offers whites the chance to transcend the tired racial divides of the past,” said an Obama campaign operative. Likewise, Mr Obama combines his foreign policy promise to “renew America’s moral position in the world” with the reminder that he went to school in Indonesia (his mother was briefly remarried to an Indonesian national) and that he has close relatives living in Kenya.

Some, including Karl Rove, President George W. Bush’s electoral strategist, who wrote on these pages last month, have mocked Mr Obama for equating a four-year childhood spell in Indonesia with an ability somehow to intuit how the rest of the world feels about America. Last year Mrs Clinton called Mr Obama “naive and, frankly, irresponsible” after he pledged to talk to the world’s worst dictators in his first year in the White House.

But when Mr Obama repeats that promise on the campaign trail it often triggers his loudest applause. Another popular line comes from his promise to stick to America’s constitution – in contrast to Mr Bush’s alleged flouting of it since the attacks of September 11.

Having been the first black editor of the Harvard Law Review and having taught constitutional law at Chicago university, Mr Obama’s pledge also resonates with his biography. “[I will be] a president who has read the constitution, who has taught the constitution, who believes in the constitution and who will uphold the constitution,” he says.

Detractors, including much of the Democratic establishment, who remain largely behind Mrs Clinton, point out that Mr Obama’s campaign may be strong on (often electrifying) oratory but it remains thin on details. They say that a vote for Mr Obama, who is only half-way through his first term in the Senate and who served for eight years in the Illinois state Senate, would amount to a “roll of the dice” and that America can ill- afford to elect someone who would have to “learn on the job” in the White House.

Some draw a parallel between the election of the relatively inexperienced – and ultimately disastrous – Jimmy Carter in 1976 after the nightmare of Richard Nixon’s presidency with Mr Obama presenting himself as an antidote to what many Americans see as the dark night of the Bush years. “What America and the world needs is a period of realism and competence in the White House,” said one prominent Clinton supporter. “Obama offers inspiration when what we need is perspiration.”

Over the next few days and weeks as the nomination circus moves to New Hampshire then Nevada and South Carolina, Mr Obama will face much more intensive scrutiny than before. Unlike Mrs Clinton, who has demonstrated her resilience over the past 15 years in the face of some of the bitterest attacks suffered by a public figure, Mr Obama is relatively untested.

In what some described as his historic acceptance speech on Thursday night, Mr Obama dangled the prospect of a once-in-a-generation opportunity to change the face of American politics and of “remaking the world as it should be”. It is an almost impossibly audacious claim. It is also one that is catching the imagination of large swaths of America. Whether or not Mr Obama can sustain it in the face of closer cross-examination, even critics would agree that he has for the time being pulled off the rare feat of capturing lightning in a bottle.