In five words, Diane Kelley managed to combine the reverential, the conspiratorial and the plain ready-to-jump-up-and-down-through-excite-ment. Standing beneath a homemade sign – “Respect. Empower. Include” – she leant over and whispered: “He’s coming here on Sunday.”

That was on Wednesday, and it was the first that the Denver campaign team knew of the Obama rally. Seventy-two hours later, 1,000 of us pack the sports hall of Manual High School. “Look at the person next to you,” the state campaign director shouts over an address system, “and say, ‘Hey, smile, you’re making history’.”

In the preceding three days, a lot had happened. A venue was chosen – Denver’s central park – a fence erected, police shifts rearranged. Scores of portable lavatories appeared in the night. It looked like the scene of a rock concert, except that Glastonbury was never organised in three days.

The most important task given to the local campaign office, though, was not to organise, but to recruit – 250 volunteers from each of the Denver districts, 250 from the rest of the state. And the volunteers’ task? For each to recruit 20 more from those attending the rally.

This is the logical conclusion of the Obama phenomenon. The talk is of hope, of change, of empowerment but, to achieve that, the grass roots must become voracious triffids, and that is the primary purpose of the rally. We are one vast, benevolent, volunteer pyramid scheme.

Sunday morning. Three hours before Barack Obama is due to speak, and the queue is already five-deep around one city block. An hour before his speech, and it takes 20 minutes to walk the length of the line. The venue’s capacity is 100,000, but it looks like some will be disappointed. Yet every person in that queue – with their Obama facepaint and their “Hopemonger” T-shirts – has been asked, in all likelihood repeatedly, to give up three hours for the campaign.

The problem is, Mr Obama may be up in the polls, but those polls rely on a lot of first-time or infrequent voters turning up this year. Databases and demographics in a central computer identify the likely Democrats. Public records tell us which of those are less likely to vote. Then we hit the phones.

I tell an elderly woman in the line that we are looking to fill 39,000 volunteer shifts in Colorado alone.

“To do what?” she asks. “You know, I got three calls in an hour last night. It’s annoying – it’s like this,” she says, pinching my arm repeatedly. “I’m voting Obama, but it’s enough to make you . . .” she considers briefly the worst outcome. “It’s enough to make you consider McCain.” She exhales, shocked by her own audacity.

Mr Obama clearly feels differently. His speech is familiar, but there are enough asides and improvisations to make us feel that we are more than just another stop on the trail. And, as the rock music fades in and the secret service prepares a corridor, he has time for one last entreaty. “If you organise with me, march with me, knock on doors with me, then I promise you: not only can we win Colorado, but we can win this election.”

The volunteer army pushes on.

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