They have a month. Senators John McCain and Barack Obama have four weeks and a day in which to persuade US voters to send them to the White House. In that time each candidate will hold carefully staged photo opportunities in diners and among the straw bales and grain silos that denote, in a single image, rural America.

But most of their appearances will be in TV studios, civic centres, high schools, and shopping malls. There is a simple reason for this. The race will not be won in small-town America. Contrary to the explanation for Sarah Palin's popularity among Republicans and some swing voters, the US is not a small-town country in terms of its built environment or even its prevailing attitudes. It is overwhelmingly urban (and suburban). The race will be won and lost in its cities.

The statistics should not be surprising, but in the circumstances they are worth rehearsing. Eighty-three per cent of Americans live in “metro areas” defined by the Census Bureau as having a population of 50,000 or more. Of these, the biggest 100 account for 68 per cent of all America's jobs and three quarters of its GDP.

Population density and diversity in these areas could prove crucial. After a generation of “white flight” from city centres to the edges of big cities, studies show a return of the middle class to downtown districts, and more ethnic variety than ever in suburbia. The largest “metros”, meanwhile, are relatively crowded. Forty per cent of those with a population of more than 100,000 have a population density greater than that of Las Vegas. But Anchorage, one of whose suburbs is Governor Palin's home town, has by far the lowest.

The political significance is clear. While Governor Palin's family story may appeal to many families, few have actually experienced anything like it. This, besides her weak grasp of policy, may help to explain the public response to her debate last week with Senator Joe Biden. Commentators of all stripes called it a win for Governor Palin because she radiated charisma and was not humiliated. But in two national polls, voters gave it to Senator Biden by a wide margin.

The dominance of urban and suburban America in the electoral process presents Senator McCain with two serious challenges. First, to prevent an Obama landslide he badly needs the support of blue-collar “Reagan Democrats” on the coasts and in the Midwest, a constituency that seemed within his grasp when it threatened to defect en masse from the Hillary Clinton camp rather than support Senator Obama. But he will not win their votes if the Palin effect proves as superficial as her debate performance and his own economic management credentials continue to be found wanting at a time of crisis.

Secondly, there is a different big city America that was, until recently, solid McCain territory. It consists of the new sunbelt “exurbs” of the South and West, from the fringes of Atlanta to the golf course communities of Phoenix or Tucson. But it was here that the US real estate bubble grew fastest before it burst and it is here that the sub-prime crisis is being felt most acutely in foreclosures, personal bankruptcies and unemployment - especially in the construction business.

Thomas Jefferson considered great cities “pestilential to the morals, the health and the liberties of man”. Many voters in 2008 agree, but not enough to decide the outcome of this race.