Two women walk out of John McCain’s Mid-West headquarters carrying a pile of voter canvassing sheets, one sports a baseball hat declaring her a “team leader” of the Republican campaign. And both are black — an unusual sight in an election where Barack Obama’s support among African Americans is almost monolithic.

Are they volunteers? They look at each other sheepishly. “Not exactly,” replies one. “We work for an employment agency,” says the other. Who are they voting for? “I don’t want to say,” says the first woman. “Obama — of course!” whispers the braver of the pair.

They laugh, then look over their shoulders at the office behind them. “Don’t give him your name, he’ll put it in the paper,” says the cautious one, explaining that they cannot afford to lose their $10-an-hour (£6) jobs. “This is embarrassing. We’re doing this because we have to live. At least none of our friends can see us. We’re from Chicago — like Obama.”

Republicans have had to hire mercenaries for this ground war. And, if the experience outside the McCain headquarters was any guide, they may not all be shooting in the same direction.

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Mr Obama, by comparison, has enough resources to spread his forces out like an invading army. In a dozen battlefield states, including Iowa, he has more than 700 offices, staffed with thousands of field organisers and hundreds of thousands of volunteers.

In the breadth of its ambition and attention to detail, the campaign of Mr Obama — he talks of it as a “movement” — may surpass a Republican grassroots organisation built for President Bush in 2004 that has since fallen into disrepair. The Democratic ground operation, together with a concerted effort to register millions of new voters, may swell support upwards of 3 per cent next week.

Mr Obama has recently expressed pride, even awe, at the power of his election machine. “We’ve been designing and we’ve been engineering and we’ve been at the drawing board and we’ve been tinkering,” he said. “Now it’s time to just take it for a drive. Let’s see how this baby runs.”

But for all the fine tuning it has received over the past year it is still stamped: “Made in Iowa”. This is where it all started for Mr Obama almost ten months ago when the discipline of his volunteers and a flood of younger voters propelled him to victory in the Democratic caucuses. On that same cold January night Mr McCain skidded into fourth place after a helter-skelter last-minute vote-chase.

And Iowa is where the contrast between the two campaigns is still the starkest. Mr Obama has 50 offices compared with 16 for Mr McCain — and four times the number of staff.

At the Obama outpost in Mason City, lights are still on at 10pm while youthful organisers plough through lists of the neighbourhood teams and precinct captains. The walls are decorated with idealistic amateur art depicting Mr Obama alongside peace signs and the mantra of the 280-page Obama campaign field manual: “Respect, empower, include”. The floors are littered with the detritus of elections: half-eaten food, a Hallowe’en pumpkin, a notice showing that a refrigerator and a toaster have been received on loan.

Among the volunteers in this office is Suren Pandita, a Labour Party worker from Croydon. “The big difference is just the sheer intensity,” he says. “Here we will have teams of five or six people for every single precinct and they will work each house where we’ve identified supporters until we’re sure they have voted. Almost every day is like election day.”

Volunteers are given a four-page script setting out their “persuasion rap”, additional talking points and exhortations to make people cast ballots early — a particular obsession for this campaign. So far, only 29 per cent of early voters in Iowa have been registered Republicans.

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