If Barack Obama goes on to lose the Democratic nomination, April 28 may well emerge as the day the tide turned against him. For on that morning, the Reverend Jeremiah Wright grabbed the nation's attention for long enough to tell it that Mr Obama was just another insincere politician.

As the man who turned Mr Obama towards Christianity and has served as his spiritual guide for two decades, one would assume that Rev Wright ought to know. Still, whatever one might make of his views, the fact is that on some topics which happen to be hot-button issues for Americans - 9/11, AIDS, Louis Farrakhan - they stand at variance with those of most of his compatriots. Most important to Mr Obama, they will probably offend many in the white working-class, a group whose support he has been struggling to attract.

Mr Obama's spectacular breakout in Iowa early this year briefly raised a possibility which exhilarated many Americans: that the country might finally move beyond its racial past and elect a 'post-racial' candidate. His charisma and message were so powerful that people who we would not normally expect to vote for a black man, including many of the good folk of Iowa, ignored past prejudices and turned out in their numbers to join what was fast becoming a movement.

The injection of race

Just when, and how, race got injected into the Democratic primary campaign will remain a topic of fierce debate. The Obama campaign, and many independent analysts, blame the Clintons for doing it. Repeated references to Mr Obama's drug use, use of his Arabic middle name, weak denials that he was a Muslim, the equation of his candidacy to that of Jesse Jackson; all these were criticised as efforts to remind voters who were not themselves post-racial that Mr Obama was, after all, black.

For its part, the Clintons blame the Obama campaign for twisting their words. But while the balance of opinion would appear to tip against the Clintons on this one, it probably no longer matters. Rev Wright has put race on the agenda in a way that may weaken the candidacy of Mr Obama.

That Rev Wright would decide to launch a speaking tour of sorts to redeem his name is understandable. So is his choice not to repudiate his more controversial positions - after all, many Americans share them. However, his explanation for differences of opinion between himself and Mr Obama was striking.

The controversy

When Mr Obama first confronted the controversy over his pastor in a widely-hailed speech last month, he maintained that despite the profound influence he had had on his life, Rev Wright held views he didn't share. Polls suggested most Americans related to that position: they all had loved ones whose opinions differed from theirs.

However, Rev. Wright implicitly rejected that statement. When asked about the difference of opinions, he hinted strongly that Mr Obama was being disingenuous, and was just distancing himself to win votes.

Almost at once, Mr Obama's poll numbers began sinking. Hillary Clinton's campaign, all but written off by pundits, has been revived.

The nuance

A furious debate has broken out among African-Americans. Many have condemned Rev Wright for appropriating the mantle of the 'black church,' pointing out that the religious experience of black America is more diverse than he suggests.His suggestion that to criticise him is to criticise the black tradition in America, if rhetorically elegant, writes many voices out of the debate.

Be that as it may, Rev Wright - inad-vertently or not - has stripped the nuance from Mr Obama's discourse on race. That may be proper or not, but Mr Obama faces a serious new challenge.