Obama by a landslide. That is what would happen if British voters got to choose the next US President. According to this week's YouGov poll on that question, Barack Obama would take 49% of the vote to John McCain's puny 14%.

Obama by a landslide. That is what would happen if British voters got to choose the next US President. According to this week's YouGov poll on that question, Barack Obama would take 49% of the vote to John McCain's puny 14%.

Hardly surprising surely? With the American economy in freefall, a disastrous, costly, illegal war in Iraq, gasoline at a record $4 a gallon and Bush's abysmal poll ratings, the November election should be a shoo-in for the Democrats. Yet, as Obama finally eased over the finish line and into the Democratic nomination yesterday, an averaging of five recent major US opinion polls gave Obama a lead of less than 2% over his Republican rival for the White House.

Why the difference? I've had some interesting dinner guests this week. On Monday, friends from the British Virgin Islands came through. "Nobody in the Caribbean thinks Obama will win," said one of them. The next day the widow of a respected CBS journalist pitched up. "I'd love to be wrong but, if my American friends are anything to go by, Obama doesn't stand a chance," she lamented. Eventually we agreed a wager. If McCain wins, I'll pay for dinner in a restaurant of her choice. If Obama wins I choose and she pays.

We all love the way the Illinois senator has energised the presidential campaign. And we acknowledge his gift for sounding positive about the future of America to an electorate now painfully aware of how their country has lost its world standing. But beyond all the rhetoric and positive vibes, one question nags us all: will Americans now, or indeed ever, elect a black man to the White House?

Nobody should generalise about the United States. As a white, English, middle-class, graduate student at Hillary Clinton's alma mater, Wellesley College in leafy New England, I became friendly with Deborah, a brilliant mathematician, the youngest of 10 children from a tenement in Roxburgh, a poor black suburb of Boston. She was also about four times my size. But if anyone thought us an odd pair, skating on the college lake in winter and swimming in it on hot summer afternoons, nobody ever mentioned it.

During the long vacation, I bought a 15-day Greyhound bus pass and headed south, staying with students along the way. The moment I crossed the Mason-Dixon line, the difference was palpable. More than a century after slavery had been abolished, the black and white communities lived almost entirely separate lives and any contact between them, however fleeting, seemed to be frowned upon. As I waited to renew my ticket at the bus station in Atlanta, my host's father advised me to move out of the queue for a black clerk. "We find the white clerks are much quicker," he asserted. Outraged, I stayed put, willing the black clerk to back me up by zipping through his queue at top speed. Gratifyingly, he did.

The incident came to mind yesterday when I looked up the Washington Post's website and some of the postings in response to its report of Hillary Clinton's final win in South Dakota. After literally dozens of racist comments, including deeply offensive personal remarks about Obama's wife, Michelle, a shocked British contributor wrote: "You guys are so hung up on race. The only reason blacks are in the USA is from slavery, and for centuries they have been treated as an underclass. This wound runs so deep it's almost genetic. Maybe voting in a black President will be a small step in the healing process." Maybe.

Those posting these foul comments wear the convenient cloak of anonymity.

Asked face to face whether they will vote for Obama, or any black candidate, the chances are that some will lie. That is why opinion polls tend to underestimate racial dimensions in voting. What we know should worry the Democrats. Questioned in exit polls in South Dakota, eight out of 10 voters said they would vote for Obama against John McCain. But in three other states that recently gave Hillary Clinton convincing wins - Kentucky, Puerto Rico and West Virginia - a majority said they would be unhappy with Obama as the Democratic nominee and only around half said they would vote for him.

We have to remember that if the Democrats had stuck with winner-takes-all voting in primaries, Hillary Clinton would have wrapped up the nomination months ago because she did well in most of the big states. Even with the current rather cumbersome proportional system, Obama only won by a short head. Now the real battle begins and if the man from Illinois is to have any hope of winning it, he must set to work on winning over the three groups who voted overwhelmingly for Clinton: male blue-collar workers, older women and Hispanics (who seem even less inclined than poor whites to vote for a black candidate).

This is not necessarily an argument for appointing Clinton as his running mate. Hillary may not want it anyway. As one former Vice-President once put it: "The vice-presidency isn't worth a pitcherful of warm spit." Appointing someone with such strong links with the past risks diluting Obama's message about "change". Besides, the essential difference between the primaries and the general election is that winner needs to carry not only his core voters but also middle-ground independents.

We may think Obama looks like a winner because in British politics age seems to be just as important as race. Look at the treatment meted out to Menzies Campbell (then 66) as head of the LibDems. Through British lenses, even Gordon Brown looks crumpled and passed his use-by date, compared with shiny new boy David Cameron. If McCain is elected he will be 72 by the time he takes office, the oldest in US history. Yet in a land where it is common to work full-time until 70, ageism barely plays by comparison with racism. It may even help John McCain to stress his experience by contrast with a man with little by way of political track record.

This is crazy. The election should be about what matters: the war in Iraq and the economy. On both, John McCain has voted solidly with George Bush, the President Americans are rejecting so emphatically. Obama opposed the war and would get American troops out of Iraq quickly. And he is committed to taking up Hillary's reins on reforming healthcare. His big speech on the race issue, on March 18, was both audacious and eloquent. It was perhaps the most important contribution to American race relations since Martin Luther King had his famous dream. It cleverly made the point that both black and white anger about race were counterproductive. Yet, ironically, it was the point in the campaign when race became a key issue. The toothpaste is out of the tube now and it can't be forced back.

In an American TV series, Barack Obama would be elected. In The West Wing, that's what happens to Matt Santos, a character partly created by David Axelrod, who is now Obama's strategist. But this isn't a TV series. This is a country where bloggers such as "Nick NYC" write: "There is no way that non-African Americans will settle for a black President." A nagging voice tells me I'll be paying for that dinner.