It is only when you stand next to it, that you realise quite how vast the US presidential election is: big in cash, big in time, greedy for attention: like the Mississippi, it just keeps rolling along, though no one can safely predict where. Yet for all its bulk and ballyhoo, the American democratic system has often hinged on the smallest things: a hanging chad here; a word out of place there; a chance catchphrase; the tiny sliver of the vote (just 0.1 per cent) that gave Kennedy victory over Nixon in 1960.

I have been trailing Hillary Clinton and Barack Obama back and forth across Pennsylvania, following the great political caravans, but at the same time jotting down the small things in my notebook, wondering which might tip the balance.

The most extraordinary primary battle in history may finally be decided by Obama's money, or Clinton's political machine, but if history is anything to go by it may be settled by something far less conspicuous: a mere handful of super-delegates, a television gaffe or some moment as yet unimagined. In the biggest democratic show on Earth, it is the small things that count.

Obama's Oratory

It is hard to describe the sheer, elemental power of the way this man speaks in public. With one hand in pocket, without notes, he almost sashays around a stage, pivoting like a boxer. The cadences roll one over another in waves, but with an internal rhythm that is close to a jazz riff. And yet, as I listened to him in full flight in a packed Pittsburgh auditorium, something went off key. The American Revolution, he declaimed, came about when the people rose up to demand “their independence from British tyranny”.

British tyranny? It seemed an anachronistic and oddly facile way to describe historical events. And then it struck me: he doesn't really mean that, but the words sound good. The rhetoric had overrun the sense. Obama is not a historian, but just for a moment I thought I glimpsed a politician profoundly, and perhaps worryingly, in awe of the power of his own voice.

Clinton's Twang

I am convinced that she used to have a strong Arkansas accent: when I first covered the Clintons campaigning in 1992, both Hillary and Bill sported southern tones that zinged and plucked like a couple of Appalachian banjos. But in Pennsylvania last week, her voice was as flat and featureless as a Midwestern potato field. This is no criticism; it's further proof of the way she has developed as a politician. Fifteen years ago she was brittle, unbending, usually on the defensive. Today she seems to adapt instinctively to her audience, like another great political chameleon: her husband.

Celebrity Wars

In the middle of another rally, Obama melodramatically paused, pointed into the crowd in unconvincing surprise, and declared: “I do believe that is Franco Harris.” A bearded man roughly the same shape as a Humvee rose to his feet and everyone screamed.

Franco Harris is an impossibly famous former football player, a receiver who once took a long pass so brilliantly it has gone down in history as the “Immaculate Reception”. The reception of the crowd was ecstatic: you could almost hear Obama's poll numbers climbing. That puts Robert de Niro, George Clooney, Matt Damon, Stevie Wonder, Oprah Winfrey and now Franco Harris in the Obama camp; while Madonna, Jack Nicholson, Steven Spielberg and the poet Maya Angelou are all behind Hillary.

Forget the super-delegates: one big superstar endorsement, at the right moment, could clinch this thing.

A Race Race

In a no-hope bar in a scrawny rustbelt town, I heard a white man utter an ugly truth, in heavy disguise. “My father,” he said, “could never vote for a man like Obama.” Replace “my father” with “I”; and “a man like Obama” with “a black man”, and you have it in black and white.

Clinton won 63 per cent of the white vote; and Obama 90 per cent of the black. Many white Democratic voters have said that if Clinton loses they would rather vote for John McCain than Obama.

Race is not the only determining factor, but it is undoubtedly one of them: America is a far less racist country than it was even a decade ago; yet 44 years after the Civil Rights Act, the prejudice is still there, bubbling evilly just below the radar, seldom openly acknowledged, draped in euphemism, disguised but potentially decisive.

Bitter Fruits

I met many small-town Americans in Pennsylvania who are “bitter” and worried, precisely as Obama said (and now fervently wishes he had not). They do “cling” to religion, and to their guns as a second faith. They just don't want this pointed out by a young black man in a sharp suit who went to Harvard. I spoke to a Republican strategist who could not disguise his glee: “You will hear the word ‘bitter' every day from now to November,” he said. Obama may never recover from telling the truth.

From a distance, this epic battle for the nomination can seem baffling. But from close up, it is a fascinating, ever-evolving mosaic, made up of tiny shifts of nuance and symbol, and a remarkable mirror to America.

There may be simpler, faster and less expensive ways to choose a candidate, but no system has yet been devised that says more about the nature of country that produced it.