Sen. Barack Obama attacked the issue of race in America head on in Philadelphia yesterday, describing the stubborn pervasiveness of bigotry, renouncing the hate speech of his longtime pastor and offering a compelling plea to move beyond long-standing prejudices.

It was a crucial test for Senator Obama, who has built his reputation as a leader and prospective president on his ability to transcend racial divisions and bring Americans together. But in recent days, he has become ensnared in criticism of his minister, the Rev. Jeremiah Wright, whose fiery sermons describing the United States and its government as racist, corrupt and murderous have provided ammunition for right-wing radio and sensational TV spots for even mainstream media. The replays of the Chicago minister's sermons were unrelenting in what bordered on a calculated bid to remind viewers of Mr. Obama's race and raise alarms about his qualifications.

But after clumsily trying to deflect the criticism from himself, Senator Obama must have realized that a denouncement of Mr. Wright's rhetoric alone wasn't going to end the questions.

In a compelling speech yesterday, the Illinois senator enlarged the subject from his choice of pastor to the toxic effects of race in America. He did it convincingly by renouncing the pastor's comments but putting them in a larger context of the continuing blight of racism on both blacks and whites. He said he could no more disown Pastor Wright than he could his own white grandmother, who loved him deeply but once confessed her fear of black men passing her on the street.

Mr. Obama turned the controversy into an opportunity to again highlight his reasons for running for president - to change the conversation and agenda in America. He compared the anger of some African-Americans who continue to feel the effects of discrimination with a similar outrage felt by most working, middle-class white Americans who have seen their jobs shipped overseas or their pensions dumped. And in uniting them, he reinforced a shared agenda to challenge a corporate culture rife with inside dealing and greed.

Whether Mr. Obama has convinced supporters and other voters of his sincerity and commitment to a new vision may become apparent in next month's Democratic primary in Pennsylvania, where polls show his opponent, Sen. Hillary Clinton, is favored by many working-class white voters. But win or lose, Mr. Obama's thoughtful exposition of race in America was an important contribution to this presidential campaign.