With the Super Bowl behind them, Americans now turn their attentions to an equally intriguing contest: Super-Duper-Tuesday, when Democrats and Republicans battle to choose their candidates for the White House in 22 states. By Wednesday, half the delegates to their late-summer conventions will have been chosen. But, as the sports commentators would put it, there is still all to play for. In a contest that has defied predictions from the start, Hillary Clinton and Barack Obama look set to slug it out for some time to come, following the withdrawal of John Edwards. For the Republicans, who began this campaign in disarray and out of favour, John McCain appears to have built up a strong head of steam. A Democratic victory is no longer a foregone conclusion.

So we have an intriguing prospect. By the end of 2008, in all probability, America will have elected a new president who is either a woman, an African-American or a man who will by then be 72. This is exciting, and there has been much dewy-eyed talk about this campaign as a mesmerising advertisement for the democratic process. Certainly, it has been more open and unpredictable than any recent contest, but it has important limitations. No candidate will reach the White House without multi-million-dollar endorsements from corporate sponsors, who expect favours in return. (This ought to put recent funding spats at Holyrood and Westminster in perspective.) This, and the sheer size and diversity of the American electorate, effectively squeezes out candidates on the left.

In addition, though there have been debates about policies, this election has been more about personalities. When it comes to presidents, Americans like them big. It has always been this way. Unfortunately, the best candidate is not necessarily the best president. Yet much hangs on this choice for all of us. The best candidate is not the one that makes the most eloquent speech or runs the most compelling television adverts but the one who can steady the wobbling American economy without betraying the sick and the poor. It will be the one who can extricate the US from its disastrous entanglement in Iraq and rehabilitate its standing in the world. And it will be the one who can turn around the American supertanker on the issue of climate change.

Barack Obama has charisma, a strong following among young and black voters and some handy endorsements, including Teddy and Caroline Kennedy, JFK's brother and daughter. However, there are real questions about whether he is experienced enough, especially when things get nasty, as they are bound to do once the Republicans get their teeth into him. Hillary Clinton, meanwhile, has bounced back from some terrible reverses, notably her botched attempt to reform healthcare and the Monica Lewinsky scandal. As the maverick who failed to secure the Republican nomination in 2000, John McCain can just as legitimately sell himself as a candidate who also represents a change from the Bush era.

Two clouds could overshadow this campaign. The "ism" that does not appear to figure in American, as opposed to British, politics is ageism. At 71 (three score years and 10, plus one), McCain is already two years older than Ronald Reagan was when he entered the White House, yet cheerfully parades his 95-year-old mother. It remains to be seen how much two other isms, sexism and racism, matter to American voters. Officially, race and gender are irrelevant; unofficially, many believe that each will count for a lot. It would be tragic if these factors, rather than competence and leadership quality, decided one of the most crucial presidential elections in American history.