Every day last week, the polls changed. On Tuesday, for example, Barack Obama was ahead by four percentage points across the USA as a whole. In the five battleground states of Florida, Michigan, Virginia, Ohio and Colorado, he was ahead in three, behind in two. By Friday, his national lead had increased to 5.7 points, and he was ahead in the five battlegrounds which will effectively decide the election. The lead in Michigan was seven points; in Colorado it was 4.4 and in Florida it was 3. Obama was ahead in Virginia by 2.4 points and in Ohio by two and although both those results are within the margin of polling error, they represent a big turnaround from just a week ago.

These are landslide figures, which point to what looks like an increasingly obvious outcome: Barack Obama will be the next president of the United States. Though his campaign has been less than inspiring after the high drama of the primaries, it has made up for its lack of goosepimple moments by being surefooted and confident. Obama convinces as a potential president; indeed, he often looks like he's president already. McCain, by contrast, is an idea-free zone, lacking in energy, vision and a confidence-inspiring vice-presidential candidate. His one key advantage over Obama – experience in the area of national security, fighting the enemy, etc – has been neutered by his choice of Sarah Palin, whose only experience of conflict seems to have been annoying her mother-in-law enough to make her think about voting for Obama.

Which is not to say we can all go to bed on 4 November confident that when we wake up, America will have elected its first black president. Obama's blackness, which has been less of an issue since the Democratic convention in August, may yet play a key role in the outcome of the election. Indeed, many political scientists, commentators and observers are worried that a phenomenon known as "the Bradley effect" will come into play over the next few weeks.

The Bradley effect is called after Tom Bradley, still the only mayor of Los Angeles to have been black, who lost an election to become governor of California in 1982. Although all the polls in the weeks before the election pointed to a victory for Bradley, the mayor lost. Writing in the New York Review of Books recently, veteran political scientist Andrew Hacker pointed to several other elections where big opinion poll leads for black candidates did not materialise on polling day.

Hacker believes many of the opinion poll results we are seeing now from various states could be reversed on election day. "Some people who are telling pollsters they're for Obama could actually be lying," he writes. "Since 1968, the Democratic Party has not been able to muster a majority of white Americans. Al Gore fell 12 percentage points behind among white voters in 2000, and John Kerry had a 17-point gap four years later." The Obama campaign should print signs to post in all its offices, he suggests. They should say: "ALWAYS SUBTRACT SEVEN PER CENT."

Those who disagree with Hacker argue that the presence of a black man on the presidential ticket of one of the two main parties is a sign in itself the Bradley effect is no longer the factor it once was. Yet in June, 20% of white people who responded to an opinion poll for ABC News and the Washington Post said a candidate's race would be an important factor in helping them to decide how to vote. Thirty per cent admitted to feelings of racial prejudice.

There are other reasons for concern. The electorate in the US is predominantly white. (In 2004, for example, 94.2 million whites voted compared to 13.5 million blacks.) Many black people who would like to vote have been disenfranchised, by being prisoners or ex-prisoners, by not having the photo ID required to vote in some states, by their tendency to move house more often than white people or through a variety of other reasons.

In addition two states, including Colorado, one of the battlegrounds, will be voting on 4 November on whether they should ban affirmative action. This brings race right into the polling booth on election day. Smooth as his passage to the White House may currently seem, Barack Obama is still a long way from making the kind of history it would be a privilege to live through and witness, even from thousands of miles away. So far, he has done breathtakingly well, quelling many doubts about his experience, his capability, his energy and his youth. The last battle, whether anybody likes to admit it or not, is about race.

In 30 days' time, we will know the outcome. I wish I felt more confident.

ddoyle@tribune.ie