Forty-five years to the day after Dr Martin Luther King looked out over The Mall in Washington and declared: “I have a dream”, Barack Obama set out to fulfil it. Standing before 80,000 people on a cloudless night at the Mile High Stadium in Denver, the son of a Kenyan farmer and a single mother from Kansas accepted his party's nomination to run for the most powerful office in the world. As he did so, he reminded Americans what the young preacher from Georgia had told them in 1963: “We cannot walk alone. And as we walk, we must make the pledge that we shall always march ahead. We cannot turn back.”

America marched ahead this week, both of its own history and of much of the rest of the world. In choosing the first African-American to lead a major party in the race for the presidency of the United States, the Democrats took a momentous step away from the poisoned past of exploitation, cruelty and bitterness and towards a post-racial society of unlimited opportunity for all. They did so, in part, because millions of Americans really have grown up to see beyond colour. And they did so because millions of others, who may yet harbour secret doubts about people who don't look like them, talk like them or live near them, have been forced to look beyond their lurking prejudices by the character of Mr Obama and the compelling storyline of the black child from a broken home who has shown that anything is possible. The Illinois senator insisted on Thursday night that “this election has never been about me. It's about you.” This is not true: it is very much about him.

Mr Obama's exceptional gifts as a politician - his intelligence, his charisma and his composure - brought America to this milestone. In the two months ahead, anyone with an interest in America's future will want to question his budget plans, his foreign policy judgment, his personnel choices and his personal detachment. But his nomination is a very personal achievement, a singular example of the meritocracy of America at work. As a result, the 2008 election is shaping up to be a referendum not so much on George W. Bush, but on Barack Obama.

The Democrats know as much and, in Denver this week, America's Opposition brimmed with a sense of possibility, but also self-doubt. After two defeats at close-run elections and with relatively recent memories of how Democrats have snatched defeat from the jaws of victory, the party of Franklin D. Roosevelt and John

F. Kennedy has been asking itself whether it will again be the party of Michael Dukakis and John Kerry. President Bush's approval ratings are at historic lows, Wall Street is teetering, house prices are plummeting, the Iraq war has grown ever more unpopular and yet Mr Obama is only neck-and-neck with Mr McCain in the polls.

As a result, Mr Obama's acceptance speech on Thursday night was strikingly defensive and political. This was not his loftiest oratory, certainly nothing in comparison with his speech to the Democratic National Convention in 2004, nor his exploration of race in America in response to criticisms of the Rev Jeremiah Wright, his one-time pastor, this year. Nor, for that matter, did Mr Obama do much to redraw the ideological battle lines between Democrats and Republicans. For all the talk of change, his Democrats sound like the old “Mommy Party” with a greater sense of social justice, a larger faith in government remedies and an urge to spend more than their more austere Republican opponents.

Mr Obama took the opportunity afforded by the television broadcast of his acceptance speech to give Americans what he had been told they lacked from so much soaring rhetoric: meat and potatoes. There was a chunky section on reform of the tax codes, a pledge to end America's dependence on Middle East oil within ten years (including a commitment to roll out nuclear power stations) and plenty of specifics on healthcare, education and pensions. Mr Obama ticked off issue after issue, constituency after constituency. There may have been tears in the eyes of the crowd at Mile High, but, by Mr Obama's standards, there were few goosebumps.

Mr Obama descended from the high ground to make his most stinging attacks to date on his rival, John McCain. He praised the former prisoner of war's military service, but then laid into the Arizona senator's record in Washington. Mr McCain was too out of touch with the strained household budgets of ordinary Americans to understand their problems and, on national security, supposedly Mr McCain's strong suit, Mr Obama questioned the famously irascible senator's “temperament and judgment.”

Mr McCain promptly showed why Mr Obama has started to get personal. The Republicans are mounting a formidable campaign, using a volley of attack ads that have successfully cultivated doubts about Mr Obama's readiness to lead. Yesterday, he wrong-footed the Democrats with the surprise choice of Alaska's Governor, Sarah Palin, as his running-mate. Americans may know more about the caribou of America's northernmost state than the short political career of the former sports reporter at Anchorage television. They may yet question the wisdom of having a small-state governor with less than two years' executive experience a heartbeat away from the presidency of a 72-year-old. Still, Mrs Palin has become the first woman to stand on a Republican presidential ticket and Mr McCain has made a shrewd and explicit move to woo the disenchanted Democrats and women voters who saw Hillary Clinton's run at the White House thwarted by Mr Obama.

The tactical outlines of this campaign have become clear. The election will be fought in about a dozen battleground states between a Democratic Party seeking to bring new voters, mostly young and black people, to the polls for the first time and a Republican Party that will mobilise its base in seeking, without any mention of colour, to prevent what it casts as a young and inexperienced opponent from snatching the presidency.

In Denver this week, the Democratic candidate's colour was often alluded to, but barely addressed head-on. The party's nervousness is such that it may have chosen a black candidate, but it does not want to make an issue of race. Even Mr Obama himself did not mention Martin Luther King by name when he accepted the nomination. But spoken or unspoken, Mr Obama has refined and redefined his “dream”. He has offered himself up as a chance for the people of the United States to breathe new life into “America's promise”, not just as a place of economic opportunity, nor as just a responsible power in the world but in terms of America's original promise to move towards that “more perfect union”.