Even before he made what might become a career-defining speech in Philadelphia yesterday, Sen. Barack Obama was ahead of his Democratic primary rival, Hillary Clinton, in party delegates and the popular vote.

But that lead was threatened by the specter of race, in recent comments made by Clinton surrogate Geraldine Ferraro, the first female Democratic vice presidential nominee, and in recorded rants by Mr. Obama's longtime minister, the Rev. Jeremiah Wright.

Ms. Ferraro said last week that Mr. Obama has achieved political success largely because he is black -- remarks that Mrs. Clinton has repudiated. Rev. Wright's anti-American, anti-white diatribes preached in the aftermath of Sept. 11 have received fresh play on YouTube and political talk shows.

So it was not surprising that Mr. Obama was asked by critics and supporters alike to account for his friendly association with the preacher. Implicit in the questioning was concern that the Democratic front-runner publicly espouses an ethic of reconciliation while turning a blind eye to divisive, intolerant speech.

With a tough primary ahead on April 22 in Pennsylvania, the Illinois senator addressed the controversy in a speech that drew upon the best traditions of American oratory.

He rejected the black minister's inflammatory comments, placing them in the context of America's conflicted racial history. He also said that his own white grandmother, and by extension other Americans, have avoided the hard work of confronting society's tragic racial assumptions and stereotypes.

"The profound mistake of Rev. Wright's sermons is not that he spoke about racism in our society," the senator said. "It's that he spoke as if our society was static."

Recounting the litany of black and white, Asian and Latino, young and old supporters who make his coalition possible, Mr. Obama challenged Americans to rise above the politics of division and cynicism.

As an example of contemporary oratory, it was stunning. As political rhetoric, it was designed to do far more than damage control and, in the end, distilled the essence of his candidacy.

If Barack Obama wins the Democratic nomination in the most unlikely campaign in American history, chances are good that his Philadelphia speech will have been a watershed moment.