Hell hath no fury like a pastor scorned. That is the lesson Barack Obama may reasonably have drawn from the latest plot twist in his extraordinary campaign for the Democratic presidential nomination. But there are more important lessons he must swiftly absorb if he is to bury Hillary Clinton's resurgent challenge.

The Rev Jeremiah Wright first featured in the US presidential race as an awkward piece of history: Mr Obama's hometown pastor had damned America and called 9/11 a case of chickens coming home to roost, and the candidate had not disowned him. In an elegant and moving speech last month, Mr Obama explained why not, but distanced himself firmly from his former mentor's views. At the time it seemed enough to keep his campaign beyond the reach of those seeking to drag it into the mire of racial politics. Not so. Mr Wright has now repeated his remarks in a series of interviews that seem calculated to torpedo the Obama campaign. They could do just that.

For nearly a year, the first black US presidential candidate with a serious chance of winning the White House achieved the remarkable feat of defining himself in entirely non-racial terms. The divisive themes and language of previous campaigns run by Jesse Jackson and the Rev Al Sharpton were conspicuously absent, even when Bill Clinton tried to introduce them as his wife's hopes appeared to fade in January. Being “post-racial” gave potency to Mr Obama's otherwise vapid central message - his promise of “change”. That message now needs re-tooling. Mr Wright's apparently vindictive intervention leaves Mr Obama no choice but to acknowledge the pastor's world view, toxic as it is, and redefine himself against it.

The process has already begun. Mr Obama called a press conference on Tuesday to disown not just Mr Wright's theories, including the notion that the US invented HIV, but the man himself. Tactically, he had little choice; polls show the Wright factor driving undecided Democrats towards the Clinton camp. Strategically, Mr Obama must turn this low point to advantage by showing that he cleaves to his own values, not those of the man who happened to baptise his daughters, and that he can be ruthless enough to cut ties with old friends when necessary. If he succeeds he will have wrested back control of his campaign. He will also have shown a vital political talent for turning crisis into opportunity.

Mr Clinton had that talent to spare. He showed it first by distancing himself from a radical black activist in what became his “Sister Souljah moment”. This could be Mr Obama's Sister Souljah moment, but only if he keeps his nerve. His denunciations of Mr Wright cannot seem - or be - merely expedient, and having made them he must rapidly recapture the post-racial territory where his message resonates.

How could he do this? One compelling strategy, rather than ignore Mr Wright's theories, would be to demolish each of them in detail and in the process present a diametrically opposite vision of himself and America. Mr Obama has the skill, and the incentive. The signs are that if he can neutralise the threat from his rogue pastor a majority of the “superdelegates” on whom the nomination depends will back him, and the McCain campaign will quietly take the race card out of play. If he cannot, he could be toast.