America was founded on a promise: religious toleration, democratic self-government, human equality, happiness. And in his pitch-perfect peroration at the Democratic Party convention on Thursday night, with biblical resonance Barack Obama summoned forth “that American spirit - that American promise - that pushes us forward even when the path is uncertain; that binds us together in spite of our differences; that makes us fix our eye not on what is seen, but what is unseen, that better place around the bend”.

But far more than words, the Democratic Party's nomination of its first black candidate for president embodies that promise. Amid all the spin, attack ads, and cynicism that accompany a presidential election, the awesome historical significance of Mr Obama's candidacy - and what it says about America's 200-year-long grapple with race - must not be overlooked.

From its founding days, America was a land of promise, deliverance and romance to the European imagination. John Donne even likened it to his naked mistress:

O my America! my new-found-land,

My kingdom, safeliest when with one man man'd,

My mine of precious stones: my emperie,

How blest am I in this discovering thee!

Central to that imagery was the natural wonder of the New World as a place of extraordinary beauty. Columbus believed that in America he had arrived at the “nipple” of the Earth, which reached closer to Heaven than the rest of the world.

To the Pilgrim Fathers, the promise of America lay in religious freedom. Sailing from the Stuart persecutions of Old England to the virgin wonders of New England in the early 1600s, the East Anglian evangelical John Winthrop took as his text Matthew v, 14: “Ye are the light of the world. A city that is set on an hill cannot be hid.” With that image of deliverance in mind, Winthrop told his fellow passengers - Christian covenanters who believed that they were setting out like the children of Israel before them - that “We will be as a city upon a hill”.

In the 18th century, political freedom was joined to religious conscience as America freed itself from the “tyranny” of King George and remodelled itself in Thomas Jefferson's words as an “empire of liberty” devoted to enlarging the happiness of mankind. The Declaration of Independence offered a remarkable mission statement. “We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain inalienable Rights, that among these are Life, Liberty and the Pursuit of Happiness.”

But all men were not equal. For all the rhetoric of liberation and regeneration that the Founding Fathers espoused, what Mr Obama called his nation's “original sin of slavery” was there at the outset. “How is it,” Samuel Johnson liked to ask of American revolutionaries, “that we hear the loudest yelps for liberty among the drivers of negroes?” Slaves had arrived in the New World via the Spanish from the early 1500s and grew steadily and monstrously in number until some 15million Africans had been forcibly transported across the Atlantic.

Modelling themselves on their republican forebears in classical Greece, slave-owning patriots such as George Washington and Jefferson saw no contradiction in combining plantation bondage with their republic of virtue. But as slavery and racial inequality became embedded in the culture of America, black abolitionists were quick to point out the farce of US freedom. “I am ashamed,” William Wells Brown declared, “when I hear men talking about the despotism of NapoleonIII... Before you boast of your freedom and Christianity, do your duty to your fellow man.”

Abraham Lincoln's 1862 Emancipation Proclamation started to do that by freeing millions of slaves. It was no accident that Mr Obama - an Illinois politician like Lincoln - launched his bid for the White House in the state capital of Springfield, “where Lincoln once called on a divided house to stand together”.

Yet the poison of slavery could not be eliminated by civil war or abolition. Instead, through schooling, employment and the Jim Crow laws, racial injustice was sanctioned by the State well into the 20th century. “One hundred years later, the negro is still not free,” was the response of one Baptist preacher in 1963. “One hundred years later, the life of the negro is still sadly crippled by the manacles of segregation and the chains of discrimination.”

And it was this celebrated speech by Martin Luther King (“I have a dream...”), delivered in front of tens of thousands before the Lincoln Memorial in Washington 45 years ago to the day of Mr Obama's own acceptance speech, that provided the inspiration for the Senator's vision of American promise. “So we have come here today to dramatise a shameful condition,” King said. “In a sense we have come to our nation's capital to cash a cheque. When the architects of our republic wrote the magnificent words of the Constitution and the Declaration of Independence, they were signing a promissory note to which every American was to fall heir. This note was a promise that all men, yes, black men as well as white men, would be guaranteed the unalienable rights of life, liberty, and the pursuit of happiness... Now is the time to make real the promises of democracy.”

The nomination of Mr Obama has made real this promise and signally confounded the living, political legacy of slavery. Yet to some within the black community, his mixed parentage and absence of slave blood negates such an easy settlement. This uncomfortable conundrum has led Mr Obama to make the case for a matrimonial connection to slavery when he spoke of his wife as “a black American who carries within her the blood of slaves and slave owners - an inheritance we pass on to our two precious daughters”.

But in so many other, more inspiring ways, Mr Obama's politics have been precisely about transcending such identity politics. Instead, his candidacy stands on an altogether grander canvas running back via King to Lincoln and the Founding Fathers to the original meaning of America. If he makes it to the White House, the original sin of US nationhood might just be absolved. Let's trust the American people deliver on the promise.

Tristram Hunt is a lecturer in history at Queen Mary, University of London

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