Some towns don't see a single ad. Wisconsin has one every seven minutes

They might be the caricature of the American tourist abroad, except Al Gore and George W Bush are doing their grand tours in their own land. They are picking only a limited number of cities, happy to pass up others that don't altogether appeal, and then they leave each of them almost as soon as they arrive.

They might be the caricature of the American tourist abroad, except Al Gore and George W Bush are doing their grand tours in their own land. They are picking only a limited number of cities, happy to pass up others that don't altogether appeal, and then they leave each of them almost as soon as they arrive.

This is how it has to be, especially in a presidential race as tight as this. It has long been clear the outcome next week will be determined by a few pivotal states. But in plotting their travels, the candidates must do more than look at whole states. They must look at the media markets within them.

Every day is about targeting the right television stations in the right communities of undecided voters. Those same stations will be carrying the bulk of the presidential TV advertisements too. But there is nothing like getting your man top billing on the local six o'clock news. The Gore campaign this week said their man will hit four media markets every day until Tuesday.

"There's at least one rule of campaigning that's now clear," says John Petrocik, a political science professor at Missouri University. "If you are on the air you will do better than if your campaign is dark." That is why when a candidate wraps up a rally, he will often disdain the reporters traveling with him, all from the national titles and networks, and lavish his attention on local reporters instead.

This imperative sometimes makes for some seemingly odd choices for campaign stops. Why was Mr Bush, on Monday, in Columbia, Missouri, a mid-size town stuck in the middle of the state? Because Columbia's TV stations cover three markets for the price of one. They serve Columbia and nearby Jefferson City, but are also viewed in St Louis and Kansas City to the east and the west.

Keeping the attention of local stations is not always easy, especially if a candidate insists on visiting a place several times. Increasingly, both candidates are tackling the fatigue by having celebrities appear alongside them. Yesterday, Stevie Wonder was expected to boost Mr Gore in Chicago.

It is the sheer size of the country that forces the candidates to trace this media market mosaic from state to state. It also derives from the system of electoral college votes, where the winner is not determined by the national popular vote but by the arithmetic of the college votes earned in a first-past-the-post manner in each state.

Not that the candidates eschew the chance for national exposure on television. Mr Gore was on the late-night Jay Leno show from Burbank, California, earlier this week. Mr Leno opened the show on Monday by asking his audience to explain why the candidates kept on coming back to his show. His is meant to be a fun programme, after all, not a political one. But everyone knew the answer.

The increased focus on local media, meanwhile, has also changed advertising strategies in this race. More than half of America's largest media markets have not seen a presidential ad, while in other markets, such as Wisconsin, viewers can expect to see one every seven minutes in prime-time.

But this year, for the first time, the ads are being tailored to individual states and even individual cities. Floridians are seeing Democrat ads warning that Mr Bush will destroy the state's coastline by opening up contracts for oil-drilling, while in Seattle viewers are being asked if they want their pristine skyline to become as smog-bound as Houston has under Mr Bush in Texas.