Meet Joe 12 Pack - an Obama volunteer guardian.co.uk

Arriving in Kansas City, Missouri, we spent some time with Russell Tucker observing a ritual that now takes place across the country every day, especially in battleground states such as this one.

Thousands of people sit together in campaign offices, union headquarters and living rooms calling up people they have never met. Thousands more troop through apartment buildings and walk the streets of suburban neighborhoods knocking on the doors of total strangers. Their numbers increase at night, when their own working days are over. Their targets are most likely to be homes, and mobile phone minutes are free. This kind of activity takes place across the US every four years - but never before on this scale. By all estimates, Barack Obama's campaign is running the largest political field operation in history.

No one can say exactly how many volunteers are now working to elect Obama. At one point, the campaign said it would mobilize 6 million by election day, but it's impossible reliably to confirm that number, or any other. Like Russell Tucker, who has come to Missouri from South Carolina, many of them have traveled from states dyed so deeply red or blue that their own votes can't change the final electoral count. They want to be where the action is, and where they can make a difference.

While Obama has far more paid staff and more money than McCain, the biggest contrast is in volunteers on the ground. FiveThirtyEight.com's Sean Quinn - who, like the Guardian team, is traveling across the country west to east, but with a special focus on grassroots fieldwork - had reached Missouri when he wrote: "Let's be clear. We've observed no comparison between these ground campaigns. To begin with, there's a 4-1 ratio of offices in most states." John McCain's campaign offices tend to be sparsely populated, according to Quinn, and they "are also calm, sedate. Little movement. No hustle. In the Obama offices, it's a whirlwind. People move. It's a dynamic bustle ... You could take every McCain volunteer we've seen doing actual work in the entire trip, over six states, and it would add up to the same as Obama's single Thornton, Colorado office."

This kind of campaigning is exhausting, inefficient, time-consuming and expensive. It also works better than anything else does - especially in the final weeks before an election, when getting out the vote is everything. In a series of well-known field studies, two Yale political scientists, Donald Green and Alan Gerber, found that personal, one-to-one appeals by volunteers are the most effective tactic for increasing turnout. Door-to-door canvassing is by far the best, producing on average one vote for every fourteen visits; personal phone calls by volunteers are the next best. Mailings, emails and calls from paid telemarketers are, by comparison, almost useless.

The same is true of robocalls, a favoured strategy of the McCain campaign. Here in Missouri, residents report receiving calls with the now familiar refrain, "Barack Obama has worked closely with domestic terrorist Bill Ayers, whose organisation bombed the US Capitol, the Pentagon, a judge's home, and killed Americans. And Democrats will enact an extreme leftist agenda if they take control of Washington." Another has Rudy Giuliani saying that Obama is soft on crime.

As part of a Republican campaign strategy that now seems to be based almost exclusively on arousing voters' fears, these calls make sense. But if the experts are right, they can't compete with boots on the ground.