The Chicago street where a dangerous secret from Barack Obama's past lurks

Nobel prize-winners from the University of Chicago are so thick on the ground on the street where Barack Obama and his family now live that even the estate agents have advanced degrees.

One of them, Stephen Parker (PhD, Egyptology,) was passing the Georgian Revival pile that is the Obama home on Saturday morning, when he saw his boss, Donna Schwan, now a thorn in the side of the Obama campaign. With snow piled up on the footpaths, they chatted in the middle of the road, watched by a team of anxious secret service agents providing round-the-clock protection to the man who could soon become America's first black president.

A self-described "Hillary Girl", Ms Schwan, who is trying to sell the plot of land next to the Obama's, muttered an oath about the secret service. "Just let them try and move me," she said. "Thank God this is still a free country." As many voters go into a collective swoon over the charismatic Barack Obama, the candidate promises to help Americans feel good about themselves and their country once again, Ms Schwan is urging a little caution. "I've been there," she says. "It's like going on a hot date with a cool guy, you just don't want to ask too many questions and ruin the dream."

What she was referring to is a simmering controversy about the nature of Mr Obama's relationship with Antoin "Tony" Rezko, a Syrian-born developer who was thrown in jail last week ahead of a major corruption trial. Chicago, the most political of American towns, is where Mr Obama and Mrs Clinton's destinies were shaped. Mrs Clinton moved away a long time ago, but she has lots of friends and supporters here. Even at the nearby Valois restaurant, Barack Obama's favourite breakfast spot and a place where Chicago's movers and shakers come to press the flesh, I bumped into a woman who went to high school with Mrs Clinton.

"I think he was more than a little naive to get hooked up with Rezko," remarked Karen Dennis, "but he is essentially a decent guy, I even know the person who bought the house he sold before he did the Rezko deal."

Born Hillary Rodham, Mrs Clinton grew up Republican in a stately brick house; today named "Rodham Place" in a posh suburb on the city's North side. The neighbourhood is as Hallmark wholesome as when she was growing up. It's a sharp contrast to the overwhelmingly poor, black and troubled areas where Mr Obama landed in the late 1980s when he started out as a young community organiser.

Whoever comes out on top in tomorrow's Super Tuesday vote, a bitter struggle to become the Democratic nominee is expected to drag on for months. There is still plenty of time for mud to fly and the back-story of the political fixer, "slum landlord" and alleged criminal Tony Rezko to become more familiar to Americans voters.

It's a story that can easily be manipulated, and in the hands of Mr Obama's enemies could soon eclipse the uplifting story his work as a community organiser and his efforts to rebuild the lives of black and Hispanic workers shattered by unemployment. The story of his efforts to get asbestos removed from tracts of public housing built in the midst of at least 50 toxic dumps may be overshadowed by his links to the moneyman behind so many Illinois political leaders. Like them, Mr Obama accepted contributions from Mr Rezko, first when he launched his political career to the Illinois Senate in 1995, and while he was going on to achieve national prominence.

Mr Obama began his life as an organiser in Chicago walking the streets of Altgeld Gardens, a blighted neighbourhood where black soldiers returning from the Second World War were housed in virtual isolation from the city. Linda Randle, 53, worked with him on the asbestos issue and now helps at campaign headquarters. "He was a good listener, he walked the neighbourhoods and talked to people about their problems," she said. "The two of us worked together and I'm pretty hot-headed, but he kept me calm with his funny quirkiness which I still see on television."

The problems of Altgeld Gardens are as bad today as when the youthful Mr Obama was here trying to get rival church and community groups to co-operate. Many of the houses are boarded up and gangs of unemployed youngsters roam the streets. The community still gets the short end of things, as Ms Randle explained.

In almost every speech he makes, Mr Obama harks back to his days as a community organiser. In a recent debate he contrasted his time spent working with the poor on the South Side with the six years Mrs Clinton spent on the board of Wal-Mart, something Democratic voters are deeply uneasy about.

Mrs Clinton snapped back at him with her first public mention of the Rezko affair, calling him a "slum landlord". Since then there has been a truce between the two campaigns, as they sought higher ground in the run-up to Super Tuesday. But as soon as Mr Rezko's corruption trial starts in two weeks' time the questions about Mr Obama will resume again, but this time before a national audience rather than tucked inside the pages of The Chicago Tribune.

Mr Obama successfully pushed back at Mrs Clinton's allegation that he represented the "slum landlord" Rezko as a lawyer.

Should Mr Obama win the Democratic nomination for the presidential race, the attack dogs of the Republican Party will be all over the "Rezko scandal". Mr Obama will be attacked for actions which though perfectly legal have left him vulnerable on one of his campaign's key selling points: his good judgement.

He now describes as "boneheaded" a financial deal he made with Mr Rezko to buy a strip of land, just 10 feet wide, from the plot Ms Schwan is now trying to sell. His difficulty is that he made the property deal at a time when his political sponsor and former close friend Rezko has been indicted on federal corruption charges, an affair that was blasted all over the Chicago media.