Several times a year, with metronomic regularity, some high-profile, wildly successful professional American sportsman is caught committing a felony.

The crimes are usually drearily familiar: assault, drugs possession, firearms offences. The incident only really becomes newsworthy when the offence is something much grimmer. A year ago Michael Vick, one of the most exciting and talented quarterbacks to play American football, was convicted of organising a dog-fighting ring in his spare time, an extramural activity that involved among other things, destroying the hapless dogs that underperformed. He is now serving time in a federal prison.

The salacious detail of the plot may vary but the backstory is often similar. The fallen hero is almost always African-American. His is a tale of a young man from a background of grinding urban misery. Thanks to a prodigious sporting talent he has managed to escape into a world of unimagined wealth and opportunity. But he never quite manages to leave the life he knew behind. His less-fortunate, less-gifted fellows from the mean streets won't let him, and he succumbs to a twisted sense of obligation not to stray too far from the criminal underclass mindset that nurtured him. He is urged, in the vernacular of the street, to “keep it real”. The intended lesson is that you can take the boy out of the 'hood but the 'hood will never really let the boy leave.

The suddenly ominous turn that Barack Obama's presidential campaign has taken reminds me a bit of this dismal immorality tale of modern American life.

The black senator from Illinois is on the verge of achieving unimaginable success for an African-American politician - the Democratic nomination for president, and quite possibly the presidency itself.

But suddenly his erstwhile buddies - in this case, the Jeremiah Wright, the former pastor of the Trinity United Church in Chicago - seem intent on dragging Sen Obama back into their recidivist clutches, condemning him to the plodding mediocrity of black political underachievement.

Lest there be any misunderstanding, let me hasten to clarify what I mean. Mr Obama is not accused of having committed a crime. Nor is there any suggestion that the Rev Wright himself is trying to encourage the senator to do anything illegal.

There is nothing in anything either man has ever said or done that remotely compares with the thuggish antics of street criminals. But in the latest controversy over Mr Wright's recent comments there are echoes of the same dispute in the broader African-American community about what kind of society America is; about whether it is possible, or even desirable for black Americans to try to assimilate themselves in what many view as a white-dominated racist system.

The peril of the Wright controversy is that it threatens to open up the very wounds of racial strife that Mr Obama's campaign is supposed to be committed to healing.

Mr Obama has always acknowledged his personal debt to Mr Wright. He was baptised by the pastor; his marriage was blessed by him, and the reverend's intellectual influence was so great that Mr Obama even used a line from one of his sermons as the title of his awarding winning book, The Audacity of Hope.

But in the last month or two it has become public knowledge that Mr Wright, in addition to his doubtless many good works, also holds some pretty radical views. He has argued that the US Government actually created the Aids virus to subjugate the African population, that the US was guilty of committing countless acts of terrorism throughout its history and that, on September 11, 2001, the US got what it deserved. A month ago, when video of these pronouncements delivered from his church's pulpit first began to circulate, Mr Obama elegantly sashayed through the controversy with a clever, but as it turned out, incomplete sidestep. Rejecting Mr Wright's wilder teachings, Mr Obama nevertheless declined to renounce or repudiate directly the man.

But this week, the media-hungry Rev Wright upped the ante. During a widely trailed press tour conducted while Mr Obama was fighting off the renewed vigour of Hillary Clinton's campaign, he repeated most of his outlandish remarks. For good measure, he also suggested Mr Obama basically agreed with him on all these issues but couldn't say so because of the realities of presidential politics.

This time Mr Obama was forced finally, belatedly, to denounce the preacher and sever his connections with him.

But the damage is potentially enormous - if not in the primary campaign that reaches yet another watershed with two key primaries next week, then certainly in the general election in November.

Mr Obama's ability to succeed where no black candidate has gone before depends on his ability to persuade most Americans that he is a different sort of African-American from the politicians they have come to know. Mr Obama has built a candidacy around, among other things, the proposition that America's deep racial differences can be transcended - that the nation's historical rift can be healed. He argues that, just as whites have to acknowledge their own responsibility for so much suffering among blacks over the centuries, so blacks must stop using historic injustice as an excuse for the high crime rates, broken family life and chronic underachievement that are the reality of existence for too many of them.

But Mr Wright's intervention reminds Americans that this is not necessarily how most blacks think. His fiery sermons may have overreached with some of their wilder conspiracy theories, but the basic thrust of what he says is doubtless shared by many, if not most African-Americans: that America is not the shining city on a hill most of its citizens believe it to be, but a fundamentally unfair place; that its role in the world is not that of principled promoter of freedom but as an exporter of misery.

This vast difference in attitudes has been opened up again by the debate about Mr Wright. Large numbers of white voters increasingly suspect that, despite his rhetoric, and given his relationship with the pastor, deep down Mr Obama might actually share that view.

His challenge now is not just to distance himself from the rantings of an eloquent extremist, but to demonstrate he really can bridge the still wide gulf in America's racial attitudes.

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