Home-made artwork covers the walls of the Obama offices in West Denver, with slogans such as “Change we can believe in” and “Respect, empower, include”.

One of the more elaborate felt-tip installations is a roulette wheel exhorting volunteers to keep calling. “Every dial brings you closer to ...” it begins — and then you spin the wheel to see what it brings you closer to: perhaps a more equitable economy or universal healthcare or an end to war.

Just below it is another poster: “Sign up to house an out-of-state volunteer.”

It is thanks to that poster that I find myself breakfasting in Colorado's most impeccably liberal apartment. My listening material is public radio and my coffee comes from a Bill of Rights mug. When the cup is filled, the rights lost under President Bush's Patriot Act disappear. I am about to spend a week with the greatest grassroots political organisation in American election history:

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Monday

Me: “Hello, my name is Tom and I work on the Obama campaign. Can I talk to you about voting by post?”

Voter: “Hey, you know what? I've already had a couple of calls explaining it.”

Me: “Awesome!” (American enthusiasm is infectious) “And have you made up your mind about the election?”

Voter: “Well, I've actually been canvassing with you guys.” Pause. “And my brother was a Democrat senator. Also, in the US we call it 'voting by mail', not 'by post'.”

Phone calls are now the heart of the Democrats' campaign. A central database selects likely voters, profiled using marketing data and demographics - then we pester them. Some people complain of being phoned three or four times a day but Americans seem far more tolerant of the intrusion than the British would be.

Tuesday

His hands an inky mess of calluses and paper cuts, P.J. Lieberman has been working at OnTarget Mailing Services, on the outskirts of Denver, for the past ten days. “I reckon I've shipped half a million leaflets,” he says with pride. “They even offered me a job.”

We are here to help him — a volunteer — and the regular paid workers with a bulk mailing to New Mexico. At top speed, 18,000 Obamas and 18,000 Bidens whizz along the production line every hour: my task is to wrap them. After my co-volunteer, the former CEO of a financial magazine, gets back troubles, my task is to stack them as well.

This parachuting in of unskilled manual labour does not seem like a sensible way to run a postal system but we aren't here for our mailing ability. We are a tax dodge with a pulse. The volunteer co-ordinator takes our photographs; not as a memento, but to prove we were there; that way, they get the non-profit mail rate. “You save us hundreds of thousands a month,” she says.

Wednesday

The campaign office opens each day at 9.30am. First in are the paid staffers, most of whom will have finished work only eight hours before. Earlier in the week an intern fell asleep in a broom cupboard. Another keeps a pillow in his car, “just in case”.

On the wall of one of the offices, between Post-it Notes and phone canvassing scripts, is a quote from JFK's campaign manager. “You can sleep in November,” it reads.

Most of the staffers are from outside Colorado, shipped in from safe Democrat, or untouchable Republican, states. They join a sizeable minority of volunteers who have taken leave and flown in from the liberal East and West Coast to help however they can. Foreign volunteers are not uncommon: I am one of three in this office from Britain, and there are two from Denmark.

One of the more enduring myths about the campaign is that it involves only young people. In fact, during the day volunteers are as likely to be pensioners. By 5pm, when telephone canvassing begins in earnest, there is no dominant demographic: businessmen fresh from the office mix with idealistic students discussing capitalist hegemonies. Others show their support with a constant supply of home-made lasagne, chilli and brownies, enabling us to work without a break until 9pm.

Today, though, the schedule is different. Mr Obama has paid for a 30-minute prime-time advert — and everything stops. A mini-documentary, it concentrates on people he has met on the campaign trail.

Initially there is jovial cynicism. One of the younger volunteers, with stubble and a baseball cap, wanders in just as a new family are being introduced. “Is that a small-business owner?” he asks, referring to one of the supposed archetypes most cited by both sides in the campaign. “I hope so, I really empathise with small-business owners.”

But by the end, as Mr Obama rises to a rhetorical crescendo, someone to my right is dabbing down sheepish tears.

Thursday

Today The Times scooped the world. We tracked down Mr Obama's aunt, living on modest means in Boston. Arriving in the office, I am unaware of our exclusive, until someone turns their radio to Rush Limbaugh, the right-wing talk-show host and liberal bête noire. Quoting directly from “The Times of London”, he devotes a half-hour segment to how “Obama the socialist, who said 'I am my brother's keeper', is letting his aunt live in a Boston slum” (he repeats the last point a few dozen times).

It probably counts as an unethical campaign contribution but today The Times bought the Obama campaign a lot of pizza.

After Colin Powell's endorsement it was only a matter of time before other American titans followed. Today we received a shipment of red, white and blue campaign badges: “Taco salads for Obama”

As well as the presidency, voters will next week give their verdict on a third of the US Senate posts. The Democrat Senate candidate in Colorado is Mark Udall, an all-American councilman who climbs mountains in his spare time. Someone donated two lasagne for us to eat on Monday. The vegetarian one was labelled “Obama lasagne”, while its meaty counterpart was “Udall lasagne”.

Friday

Elvis greets me outside the early voting centre. “Uh-huh, vote Obama,” he says, before an elderly lady shuffles by dressed as a witch.

There had been some discussion about whether canvassing would be easier at Hallowe'en because people are expecting unsolicited calls, or harder, because they only want the sort of calls that come in costume. Someone suggested that we all wear McCain and Palin masks.

John Jenkins is wearing a lumberjack shirt, a cowboy hat and a long, straggly beard. But, for West Denver's top canvasser, that is normal attire. He is taking me around his beat, one of the poorest neighbourhoods in Denver, where he has been pounding the streets since April.

“It feels like City Hall has stopped caring about these people,” he says, passing a rudimentary trailer park. “This is dirt poor. This is what it is like without a safety net. If they lost their jobs, most would be homeless in a month.”

The area is a mix of immigrant communities. El Mexicana minimart sits beside the New Saigon restaurant and Paul Lopez, the local councilman, proudly tells me that they have a mosque and a Buddhist temple. His card is translated into Vietnamese.

Opposite his offices, a shabby bungalow flies the US flag at half mast. Mr Jenkins tells me that they are probably a “gold-star family”, with a son killed in Iraq or Afghanistan.

“It's my job to get all these people here to vote,” he says. “I don't pretend that with Obama things will change overnight: I know expectations are too high. But after eight years of Bush, we can't let our country down for another term.”

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