If John McCain wins the US presidential election this November, he will be older than Reagan was in 1980. If Hillary Clinton is the successful Democratic choice, she will become the first woman to occupy the Oval Office. If Barack Obama becomes President, he will be the first African-American to achieve that honour.

Three possibilities for a historic outcome; three cliches. That does not mean they are unimportant. Whatever the result, its significance would be profound. Even in the 21st century, the assumption persists that age, gender and ethnicity are problems that campaign strategists could live without.

The strategists believe it (though they must not say so) in their turn because they harbour dark suspicions about voters, prejudices and what people really think. If the voters have decided otherwise, finally, American society and politics have changed utterly.

Cliches, nevertheless. America's public has lived with the repetition of unalterable personal facts for months now. McCain's career, stretching all the way back to a Vietnamese prison, is a well-told story. Clinton, as First Lady and senator, is hardly a new face. The Obama phenomenon has been primed to explode since the 2004 Democratic convention. The historic possibilities of November have, if you like, been discounted. They do not drive the unfolding campaign.

That remains an old-fashioned affair. Unspeakable amounts of money, rancour, smears, hard-headed strategies and soaring rhetoric compete amid a system of primaries, caucuses and delegate meetings that even Americans have troubling explaining. It tends to make a mockery of received opinion.

Not six months ago, certain knowledge had it that McCain was dead and buried. This week his dominance of the Republican race is such he is deemed worthy of a smear, with insinuations of an extra-marital affair. Half a year ago, equally, Clinton was "unstoppable", with a matchless political machine, $100m and a husband who knew, better than anyone, how elections are won. This week she has been begging for donations, portraying herself as the underdog and hinting - possibility to stiffen supporters' spines - at the possibility of defeat.

When the race began, Obama was, at best, one to watch for the future. Then Iowa persuaded received opinion of a bright new hope. That notion fell apart in New Hampshire. Obama fought Super Tuesday to a draw, on paper, but he failed in places that, traditionally, a Democrat has to win. He had Teddy Kennedy's backing in Massachusetts, but lost there. He lost badly in New York and was seen off, despite some predictions, in California.

This turned out to be a candidate as smart as his backers claimed, however. A strategy of sweeping up delegates in smaller states, many of them places that rarely vote Democrat in a general election, kept the arithmetic better than respectable. Obama also understood the primary calendar. The contests that followed Super Tuesday were always among his better bets. With 11 now in his column, he has the fabled momentum. Clinton knows it.

There is an air of defiant desperation about her statements. She is being outspent in Wisconsin, it was alleged, by four dollars to one. She invokes her experience repeatedly, and speaks the truth on that, at least where Obama is concerned. No use. Bill Clinton, the legendary campaigner, has meanwhile seemed oddly off-key, forgetting voters might be more impressed by a candidate who has no need of a husband's help.

She attempts, time and again, to tell the public that Obama is a well-cut but empty suit. Who can blame her? In policy specifics he and his rival do not differ seriously.

Iraq has given way, however, to impending recession as one of those "defining issues" of US politics. So Clinton asks who better understands "the economy, stupid" after her husband's battles on that terrain, the eight years she spent in the White House and her service since in the Senate. Clinton's essential point is simple. Obama talks endlessly about "change", much less about precise changes.

He deploys those other shining words, youth and hope - words that allow people to believe without thinking too hard. She would like voters to understand that this inspirational rhetoric is close to deceit. Instead, increasingly, it appears that those voters would rather just believe.

The women, older Democrats and working-class Americans who were her earliest supporters appear to be drifting away. The Latino vote, the Clinton "firewall", seems a little less reliable than before. The perception of a Clinton dynastic claim might finally have begun to alienate the previously undecided. Yet for the candidate, that may not be the worst of it.

When I see and hear Obama my first thought is not, necessarily, of an African-American. Certainly there is a sense of history, but not of US history. Instead I witness a charismatic, forty-something lawyer and family man, advertising his youth and unabashed about his relative inexperience, talking eloquently about the desperate need for change, of reaching across party boundaries, of new generations and aspirations. None of it is too specific. But after long years of deep-dyed conservative government his party, and perhaps his country, is eager for this story.

No prizes, obviously. This is Tony Blair, 1997. Unless there has been an unheard-of coincidence - and no, I do not overlook the significance of a black presidential candidate - the Obama strategy is the Blair strategy a decade on. What matters to Hillary Clinton, if she has not already guessed, is that in Britain, in 1997, a combination of empty eloquence and a demand for change was a stunning success.

Let's say this week's received opinion is right. Clinton needs not only to win but win well in Texas and Ohio on March 4, with Pennsylvania to follow. Most reports from the US lean towards the belief that it is already too late. Obama, they say, is sucking away the blue-collar households, with Latino families beginning to drift. One game is therefore concluded. Obama will face McCain.

Polls in nations of 300 million people, where many have no party allegiance and a great many do not care to vote at all, are on the wrong side of infallible. A recent Gallup attempt, nevertheless - "undecided voters not recorded" - might do for chicken entrails. McCain v Clinton: 49% to 48%; too close to call. McCain v Obama: 46% to 50%; good for the Democrat but not, not yet, near good enough.

McCain is the maverick, so-called, who will claim the Republican nomination despite the wishes of "values conservatives" and the rest. But even those who doubt or despise him will rally come November. Yet if polls are close to being remotely accurate, the idea that a Democrat must inevitably win the White House after the Bush years is still another piece of mere received opinion.

And if the Democrat is Obama? I remain to be convinced that racism no longer has an influence in American politics: put it no higher. I in no way insult the majority of the people. But in a tight race, a barbaric minority - 5%, 10%? - cannot be discounted.

If that is so, Obama will need better, vastly more substantial and still more eloquent words to convince decent souls to find the polling booths in November. At the age of 71, John McCain is nothing if not serious.