New England, where I started my journey, and west Texas, where I ended it five weeks later, could be in two entirely different nations. Not only had the topography, climate and architecture radically altered, but so had the people and their attitudes towards everything from religion and government to taxes and guns.

In that time the political landscape had changed too. I left the Democratic challenger's home town of Boston with John Kerry fighting to defend his purple hearts and stay in the race as he languished eight points behind Bush in the polls. I left Mr Bush's home town of Midland with the contest tied and Mr Bush explaining a looted weapons cache in Iraq.

One of the few things that has remained constant while on the road has been the ubiquity of the stars and stripes. The national flag billows everywhere. It flies from porches, hangs from store fronts and decorates the bumpers of many cars ahead of me. The interstate highway, network television and chain stores aside, the ever-present national flag has been the one constant indicator that I have remained in the same country all along.

But these demonstrations of patriotism offer little or no suggestion of which side of the political divide people are on. You are as likely to find them among Republicans as Democrats. In normal times this strong sense of national identity is the thread that keeps this diverse patchwork of states, cultures and ethnicities together.

Frayed

But these are no normal times. Indeed, over the last month it has occasionally felt as though these threads may be becoming perilously frayed. For at its heart this election has highlighted the thorny, divisive issue of what that flag stands for. For many, at stake is not just who will run the country, but who owns it and what core values should underpin it.

On the left are those who believe the nation is being transformed by a corporate theocracy. Trekking through the suburbs of Derry, New Hampshire, Pam and Patrick Devaney overcame their shyness to go knocking on doors in search of progressive voters. "I'm not comfortable doing this but it has to be done," said Pam. "Our democracy is at stake. This is the most important election in my lifetime."

"I always thought someone else was out there doing the job for us," said Patrick. "Now I wonder what we're doing in New Jersey."

On the right are those who fear the encroachment of secular liberalism. "I fear for this country if Kerry wins," said Burton Kephart, from Franklin, Pennsylvania, whose son Jonathan was killed in Iraq. "God has a plan for the ages. Bush will hold back the evil a little bit. He is a God-fearing man. He believes in praying to a God who hears his prayers. He's a leader."

Many Americans, of course, lie in between these two extremes. Like the hotel worker in Dearborn, Michigan, weighing her opposition to abortion with her opposition to the war who was rooting for Mr Kerry with reservations, they do not fit easily into either camp.

But when the nation goes to the polls today they will only have two camps to choose from and what little common ground there may have been between them has effectively been torched.

Watching the third presidential debate with about 40 students in Iowa City, the Republicans sat on one side and the Democrats on the other. At moments the Republicans would break into cheers or laughter at a phrase or facial expression of one of the two candidates, to the bewilderment of the Democrats. A few minutes later the Democrats would do the same, leaving the Republicans similarly confused.

They were not just watching the candidates on a split screen. They were viewing the entire event as though from a split screen, each side hermetically sealed from the other as though they were witnessing two completely different events in a parallel universe.

On these rare occasions when people are presented with the same raw data, the two camps have managed to fashion conclusions that are not just different but almost entirely contradictory. So rather than partisan arguments adjusting to take account of reality, reality is altered to suit the argument.

So it has been throughout the trip, with both sides rejecting negative polling results as rigged and denouncing the media for being biased in favour of the other side, leaving few basic facts that anyone can agree on.

A recent poll, released by the Programme on International Policy Attitudes, showed that the overwhelming majority of Bush supporters still believe that Iraq had ties to al-Qaida or the September 11 terrorist attacks and had weapons of mass destruction or a programme to develop them. They also believe that the world favours a Bush victory. In each case only a minority of Democrats shared those views.

It follows that from this different understanding of the problems come entirely polarised conclusions about the solutions.

Rick Sapareto in New Hampshire supports Mr Bush and the war in Iraq. "I'm very concerned that my boys may end up fighting a war in 15 years because we failed to take action," he said.

Lisa O'Neill, who lives just a few minutes away, supports Mr Kerry and opposes the war for almost entirely the same reason. "I have an 11- and 13-year-old who could be drafted if this carries on," she said. When I called them both the day after the first debate each one thought their side had won.

The Democrats that I have met seem much more aware than Republicans that the world will be watching nervously today. Indeed Republicans seem quite bullish in their indifference. But while the rest of the world has been watching the US these past few months, the US has not been particularly interested in the rest of the world.

In his classic book, Democracy in America, the 19th century French intellectual Alexis de Tocqueville wrote: "All the domestic controversies of the Americans at first appear to a stranger to be incomprehensible or puerile, and he is at a loss whether to pity a people who take such arrant trifles in good earnest or to envy that happiness which enables a community to discuss them."

This is an election about America and its obsessions, old and new, and many of them are indeed incomprehensible. Guns, gays, God, abortion, stem cells, jobs, health care, social security and the shortage of the flu vaccine have all been raised at one time or another.

"You'd think with everything else going on in the world they'd have something better to worry about than gays getting married," said Ann Fuhrman, a lesbian living in Springfield, Missouri.

But the United Nations, global warming have not come up once; the Middle East is a big issue for Arab-Americans and Jews everywhere, but nobody else has mentioned it.

Invasion

So, when they discuss Iraq and the war on terror they do so in terms of the human and financial costs to America. If the occupation were going well you do not get the impression that the invasion of a sovereign country and the lack of weapons of mass destruction would be a major issue.

"I know we're making an attempt to help the Iraqi people," said Yvonne Shostack, in Derry. "But I thought things would be more resolved than they are. People are still getting killed and we're still out there."

But what is most bizarre about this polarisation is that all the emotional energy appears to be concentrated around one pole.

Over the past five weeks I have not met one person who had a passionate word to say about Mr Kerry one way or the other. Though I don't doubt that some exist, the Democrats I have encountered are primarily motivated by their hatred of Mr Bush. "If they put up a vacuum cleaner against Bush we'd vote for it and just ask them to change the bag every now and then," quipped Gene Lyons, a columnist for the Arkansas Democrat-Gazette.

Republicans are adversely galvanised by their love of Bush. "He stepped up to the plate and showed the world what we can do," said Chris Paxos, at a job centre in Canton, Ohio.

This is not an election about platforms or ideas. It has brought the issue of who owns America and what its values should be into stark relief. But it cannot answer them.

Today will be a referendum on one man - George Bush - and his record. Whoever wins will do so by a narrow margin and inherit a deeply divided nation. But, legal challenges notwithstanding, that will be tomorrow's story. And nobody is looking that far ahead.