“I can no more disown him than I can disown my white grandmother,” he said, “a woman who helped raise me, a woman who sacrificed again and again for me, a woman who loves me as much as she loves anything in this world, but a woman who once confessed her fear of black men who passed by her on the street, and who on more than one occasion has uttered racial or ethnic stereotypes that made me cringe.”

The speech, delivered before a small audience of local supporters, elected officials and clergy members at the National Constitution Center, was broadcast live on cable. His words were directed at a variety of constituencies, including the superdelegates who, it now seems likely, will decide the nomination, and white voters in states like Pennsylvania that could be vital both in the primary and in the general election.

Mr. Obama stayed up well into the night writing much of the speech himself, aides said. His words carried familiar strains of the biography he wrote more than a decade ago about his search for racial identity.

In recent days, televised images of Mr. Obama have been accompanied by old sermons of Mr. Wright delivering a blistering critique on white America. As the days go on, it remains an open question whether the images of the pastor will fall to the side and Mr. Obama’s campaign message regains its prominence. This speech was a political risk, his advisers said, whose wisdom may not be fully known for months.

“It was very dicey at a time when race is misunderstood by some and overplayed by others,” L. Douglas Wilder, the mayor of Richmond, Va., who was the nation’s first elected black governor, said in an interview. “It was a very, very difficult subject to bring up. It had to be approached in a way that was really something of substance.”

Mr. Obama indicated that he wanted to move quickly to other issues, scheduling two high-profile speeches over the next two days on topics other than race.

“I suppose the politically safe thing would be to move on from this episode and just hope that it fades into the woodwork,” Mr. Obama said. “We can dismiss Reverend Wright as a crank or a demagogue, just as some have dismissed Geraldine Ferraro, in the aftermath of her recent statements, as harboring some deep-seated racial bias.”