Washington —- With controversy over his pastor's racially divisive sermons threatening his presidential campaign, Barack Obama tackled race in America head-on Tuesday in a defining speech that drew instant comparisons to John F. Kennedy's 1960 address about his once-controversial Catholicism.

Obama, speaking in Philadelphia, called slavery America's "original sin" and said mistrust between blacks and whites goes both ways. He said racial resentments have long shaped the nation's political landscape but that he thinks the nation is changing and that he could help heal the divide if elected.

After campaigning for more than a year as an African-American who transcends race rather than lets it define his politics, Obama had no choice but to embrace the subject. Race has surged recently into prominence as a campaign issue, and Democratic voters in some states show increasing signs of voting along racial lines.

"I suppose the politically safe thing would be to move on from this episode and just hope that it fades into the woodwork," Obama said, referring to the firestorm that erupted over racial remarks by his longtime pastor, the Rev. Jeremiah A. Wright Jr. "But race is an issue that I believe this nation cannot afford to ignore right now."

Wright, who is retiring as pastor at Trinity United Church of Christ in Chicago, drew fire in recent days after TV and the Internet played up some of his past radical statements angrily condemning America for its racism, among other things.

Obama said that while he rejects Wright's controversial statements, "that isn't all that I know of the man. ... He has been like family to me. ... I can no more disown him than I can disown the black community."

Instead of confining political discussion of race to such polarizing extremes, Obama said, America should elevate the conversation and focus on how to build a more united nation, where people of every background can build better lives and tackle together problems such as health care, education and jobs for all. He said his belief that he can help the country do that is at the core of his presidential candidacy.

Even before controversy erupted last week over Wright, race was rising as an issue that Obama would have to confront. While he pulled heavy shares of the white vote in many early voting states, such as Iowa, increasingly his support among whites has declined in states with cultural histories marred by racial friction.

Earlier this month, white voters favored Hillary Clinton over Obama by a 3-1 margin in Mississippi and by almost as much in Ohio, while black voters went for Obama by 9-1.

The next big Democratic contest comes April 22 in Pennsylvania, where the Democratic electorate resembles Ohio's, with a large white working class outnumbering African-Americans.

Obama spoke directly to both groups in his speech.

"The fact that so many people are surprised to hear that anger in some of Reverend Wright's sermons simply reminds us of the old truism that the most segregated hour in American life occurs on Sunday morning," he said.

"In the white community, the path to a more perfect union means acknowledging that what ails the African-American community does not just exist in the minds of black people; that the legacy of discrimination —- and current incidents of discrimination, while less overt than in the past —- are real and must be addressed," Obama said.

He said some whites are angry, too, because they feel blacks are often given an unfair advantage through affirmative action.

"When they are told to bus their children to a school across town, when they hear that an African-American is getting an advantage in landing a good job or a spot in a good college because of an injustice that they themselves never committed, when they're told that their fears about crime in urban neighborhoods are somehow prejudiced, resentment builds over time," he said.

"If we walk away now, if we simply retreat into our respective corners, we will never be able to come together and solve challenges like health care or education or the need to find good jobs for every American," he said, drawing a rare burst of applause in a somber address.

Clinton, Obama's chief Democratic rival, said she was glad Obama had given the speech.

"Issues of race and gender in America have been complicated throughout our history, and they are complicated in this primary campaign," said Clinton, also campaigning in Philadelphia. "There have been detours and pitfalls along the way, but we should remember that this is a historic moment for the Democratic Party and for our country. We will be nominating the first African-American or woman for the presidency of the United States, and that is something that all Americans can and should celebrate."

Kennedy's 1960 speech effectively put to rest growing questions about whether a Catholic could serve as president. Will Obama's speech prove similarly pivotal?

"The speech was effective, thoughtful and quite well done," said Bruce Ransom, a politics professor at South Carolina's Clemson University. "But there will be those who will not be convinced. They will want to know why he didn't leave [Wright's] church, why they are still friends, why he didn't condemn the minister more strongly. The speech will not quiet the chorus."

But Dante Scala, a University of New Hampshire political science professor, said that Obama's decision to reject Wright's comments but not disown the minister might earn him some respect among skeptics.

"It might be viewed as a gutsy, stand-up thing to do," he said.

The Associated Press contributed to this article.