The trouble with aspiring to a post-racial presidential candidacy is that the racial realities of America still exist.

Segregation (de facto rather than de jure), inequality and distrust persist - hardly surprising when you think how recently racism was legal and how much cultural, social and economic capital has been invested in it.

So the fact that Barack Obama has a race problem should hardly come as a shock. The fact that he would have a race problem in north-west Roanoke, the black part of town, however, offers pause for thought.

The issue here is not that African-Americans are not excited about this election or Obama's candidacy - they most definitely are. The enthusiasm is so close to the surface that you can pretty much stop any black person of pretty much any age or class on the street and talk to them about the election. You don't have to ask them who they're going to support. It's understood.

The issue is that some of the city's black leaders feel the Obama campaign is unnecessarily alienating members of the black community because it is ignoring their local knowledge and connections. "I don't think it's intentional," explains Virginia House delegate Onzlee Ware, who has a good relationship with the local campaign. "But sometimes intentions are not really the issue. Some black people have felt a little left out."

This is serious. If Obama is going to win a state like Virginia, which is 20% black and 100% southern, it must win cities like Roanoke (which is 25% black) by a significant margin. It simply cannot do that without the black vote. If African-Americans have a problem with the Obama campaign, then the Obama campaign has a problem.

Ware, and some other black elders in town, have taken it upon themselves to use their own funds to mobilise the black community, fearing that a failure to do so might result in Obama failing to reach his full potential come election day.

"The older crowd just don't connect with a 20-year-old white person telling a 50-year-old black person what to do," explains Ware. "Especially how to run their own communities. And then they get discouraged and go back to their communities."

Whether this represents a generational tension between a local black leadership demanding a gatekeeper role and a national campaign that believes that role is no longer necessary is a moot point. By the time we have worked that out it will no longer matter.

"This is a historic moment," says Ware. "And I don't want to sit down on November 5 and have anybody say it was on me ... We don't have time for no pity party."

The essential problem is rooted not in race but the dislocation between the rhetoric and reality associated with Obama's campaign. The campaign draws on the energy, commitment and engagement of huge numbers of volunteers - many of whom are working every spare hour to get Obama elected. In that respect, the grassroots nature of his support is very real and central to the strength of his organisation.

That has lead some people to mistake it for a movement, when in fact it is nothing of the kind. It's a professionally run electoral machine where orders come from the top down. Those who are involved do not have any say in policy, strategy or direction. They do as they are asked. That is not a criticism. It is a description.

The question is how the campaign that has arrived with orders from elsewhere gels and connects with the political structures and cultures that existed before it got there and will remain once it is gone. Where race in Roanoke is concerned, that is no straightforward task.

For a small southern town with a sizeable black community the town got through the civil rights era pretty much unscathed. As most people, both black and white, tell it, the city's black elders had a meeting with the city's elected officials and they decided to desegregate. And so the signs came down. There was no marching, sit-ins, hoses, demonstrations, heads cracked, jails filled or lives lost.

The benefits of this are clear. The trauma of open confrontation that blighted so many southern towns during that time was not a factor here. The transition from apartheid to normality was handled with the kind of businesslike determination that is rare for an issue of that magnitude.

But there were other things that came with the struggles of the civil rights movement that Roanoke lacks. Just two hours drive away in Greensboro, North Carolina, a sit-in by four black teenagers in Woolworths galvanised an entire community.

A few years ago, Franklin McCain, one of the youngsters involved, told me: "On the day that I sat at that counter I had the most tremendous feeling of elation and celebration. I felt that in this life nothing else mattered. I felt like one of those wise men who sits cross-legged and cross-armed and has reached a natural high.

"Nothing else has ever come close. Not the birth of my first son nor my marriage. And it was a cruel hoax, because people go through their whole lives and they don't get that to happen to them."

Black Roanoke never had that moment: a collective period of resistance, struggle and defiance. Consequently white Roanoke was never truly confronted. The city's political class successfully negotiated so that African-Americans could eat where they wanted. But that didn't mean black people could afford everything on the menu.

According to the census, the median household income of a black family in Roanoke is less than three quarters than of a white family; black families are almost three times more likely than white families to earn less than $20,000 a year, and almost five times less likely to earn more than $150,000.

Nonetheless, black Roanoke has had many other moments. For 17 years, from 1975 to 1992, the city had a black mayor, Noel C Taylor. "If you look at the number of African-Americans we have elected to political office, for a town this size, it's very phenomenal," says Ware.

So white people in Roanoke have already shown that they are comfortable voting for black politicians. And local black leaders clearly know how to mobilise black voters. The question, with less than three weeks before polling day, is whether the Obama campaign can harness that knowledge and make it work for their candidate on a scale yet to be seen.

"We know how to do this in this area because we do it all the time and we know who the people are," says Richard Chub, a 72-year-old former principal and black political activist.

"People say they're going to vote but you have to get them to the polls and for that you need to know who they are."