The coming clash between Barack Obama and John McCain is, the pundits tell us, a struggle between two Americas: liberal and conservative; black and white; young and old. But it is also a confrontation between two very different cities - Obama's Chicago and McCain's Phoenix - and their richly opposing political traditions.

As Upton Sinclair so brutally, brilliantly chronicled in The Jungle, Chicago, Illinois - nicknamed the “Windy City” just as much for its politics as the icy blasts tearing off Lake Michigan - has long been a crucible for US socialism. Together with Philadelphia and Baltimore, it was the premier city of organised labour and radical activism in the late 19th century. At times, this politics spilled over into violence: the origins of the labour movement's May Day celebrations are to be found not in Clerkenwell or the Left Bank, but in an 1886 riot in Haymarket Square that led to bombs being hurled, policemen killed and the execution of the left-wing ringleaders.

In the last century, Chicago became a bastion of the Democratic Party with its celebrated city boss, Mayor Richard J. Daley, securing the White House for John F. Kennedy with votes culled from the local cemeteries. As a city, it stands in the progressive European tradition - high-density living, mass public transport, strong public sector unions, a pioneering tradition of social work and an attractively cosmopolitan feel (with more Poles than Cracow). All of which has been masterfully chronicled by its unofficial laureate, Studs Terkel, the former communist blacklistee and godfather of American social history.

Yet modern Chicago's most famous resident is the talk-show goddess and Democrat stalwart Oprah Winfrey. For this is a city of stark racial polarities, thanks to Mayor Daley's 1960s zoning strategies that cordoned off the African-American community into vast “projects” to the west and south of downtown. And it was these wretched, ignored, unfunded ghettoes that provided the political base for Jesse Jackson, the Nation of Islam leader Louis Farrakhan and then a young community organiser called Barack Obama.

For despite the Hawaiian upbringing and Harvard degree, it is the politics of Chicago that has dictated Mr Obama's thinking. His liberalism is not some East Coast effete affair, but a progressive, cosmopolitan street-savvy ideology that has emerged from his South Side activism: a belief in the power of the State; a strict adherence to racial, social and sexual equality; opposition to guns and the death penalty; a commitment to the capacity of church and community to change life chances; pro-Palestinian and, crucially, anti-war.

But Chicago is a complex, unexpected city. For not far from Obama's Hyde Park house stands the University of Chicago - a remarkable institution of terrifying academic rigour where I spent an industrious few terms as an exchange student. Once home to Friedrich Hayek, Milton Friedman and the so-called “Chicago boys”, it was the intellectual engine house of supply-side economics, free-market policies and what became the Reagan-Thatcher revolution. Every political principle the city of Chicago stood against roared forth from the university's iconoclastic economics faculty. And while such conservative nostrums were studiously ignored in Cook County, Illinois, they found a warm reception in John McCain's adopted state.

For in contrast to the unionised, left-wing milieu of Chicago, Arizona is the land of Barry Goldwater, less government is good government and a consciously Wild West, libertarian ethos. Joyfully, there are no motorcycle helmet laws in AZ. And if Chicago grew out of the European civic model, Arizona's capital, Phoenix, is a template for postwar US urbanism: born of the airline industry and air-conditioning (which made desert living possible), it is a sprawling megalopolis of low-density housing, car dependency, and monotonous strip-malls stretching into the xeriscape.

“There are no centres, no recognisable borders to shape a sense of geographic identity,” writes the New York Times columnist David Brooks of Phoenix and its ilk. It is a polycentric universe where the rhythms of the day are orientated around drives to the shopping mall, gym, church or work. In contrast to the great railway stations and art galleries of Chicago, there isn't much downtown or inner city; few civic landmarks or historic signifiers. Through Phoenix's boomburbs, Wallgreen's follows Burger King follows K-Mart follows Starbucks. I lived for a year in this exurban terrain of freeways and drive-thrus and at least once a week I would get lost trying to find my home through the sprawling, anonymous cityscape.

As such, it is a profoundly individualistic terrain lacking Chicago's engrained social fabric of class, race or community. Instead, its churning cycle of new residents live out the American dream with no time for local taxes, planning laws or local activism (outside the often evangelical churches). Brooks celebrates Senator McCain's exurbia as “a conservative utopia” and it was these self-contained, often gated “communities” that delivered the White House for George W. Bush in 2004.

For if industrial cities such as Chicago were the breeding ground of progressive politics, exurbia represents the amorphous heartland of modern conservatism. A study by The Los Angeles Times revealed that 97 of the 100 fastest-growing communities in America supported President Bush, providing him with a decisive 1.72 million vote advantage over John Kerry. Unfortunately for Mr Obama, this conservative majority has grown, thanks to hundreds of thousands of Americans moving from the cold northern states to the southwest sunshine of Las Vegas, Arizona and Colorado - and bringing with them a remarkable fertility rate.

Those decamping to the zoomburbs are choosing to buck the US birthrate by consciously raising large families. Who then vote Republican. According to analysis by Steve Sailer in The American Conservative, the 19 states with the highest white fertility rates went Republican in 2004. John Kerry, on the other hand, carried the 16 states with the lowest rates of conception.

So here is Mr Obama's urban conundrum. For all his love of metropolitan, liberal Chicago, it is grumpy old John McCain's Phoenix that represents the psephological future. And sooner or later, Mr Obama will have to join those tens of thousands of his Illinois compatriots swapping the icy winds of downtown Chicago for the sprawling embrace of metropolitan Phoenix, “Valley of the Sun”. His future job depends on it.

Tristram Hunt is a lecturer in history at Queen Mary, University of London

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