U.S. presidential candidate Barack Obama will seek to quell a controversy over inflammatory rhetoric by his former pastor in a speech on Tuesday on the issue of race.

Flare-ups over race have roiled the campaign trail as Obama, who would become the first black U.S. president, battles for the Democratic nomination with fellow Sen. Hillary Clinton, who would be the first woman president.

Obama said the controversies have become a distraction to his campaign as he vies to become the nominee to face Republican U.S. Sen. John McCain in November.

An aide to Obama, whose speech will be delivered at a historic building across from Philadelphia's Liberty Bell at 10:15 a.m. EDT (2:15 p.m. British time), said the speech will have a strong personal element. Obama worked late into the night on Sunday drafting it.

Of particular concern is the uproar over comments by Rev. Jeremiah Wright, who served as the pastor at the Chicago church where the Illinois senator was a member for 20 years.

In sermons widely circulated in the media, Wright has called the September 11, 2001, attacks retribution for U.S. foreign policy, cited the U.S. government as the source of the AIDS virus, and railed against a racist America.

Obama has denounced the comments and Wright resigned from his role as spiritual adviser to the Obama campaign. Obama likens the pastor to an "old uncle." Wright married Obama and his wife and baptized their two children.

In an interview on the PBS program "Newshour with Jim Lehrer," Obama said the controversy threatened to overshadow his message of trying to transcend racial divisions.

"I would say that it has been a distraction from the core message of our campaign. I think part of what has always been the essence of my politics, not just this campaign, but my life is the idea that we've got to bring people together," said Obama, who often talks of his upbringing as the son of a white woman from Kansas and a black man from Kenya.

The Clinton and Obama camps have accused each other of injecting race into the campaign.

Last week, Geraldine Ferraro, a Clinton supporter and 1984 vice presidential candidate, attributed Obama's lead in the Democratic race to his being black.

Others took offence when Clinton's husband, former President Bill Clinton, in January compared Obama's victory in the South Carolina primary to success there by Jesse Jackson, a black candidate who ran for president in 1984 and 1988. Critics saw the remarks as a bid to marginalize Obama as a candidate for only black America.

But Bill Clinton told television interviewers on Monday it was a "myth" that the Clinton campaign engaged in racial politics in South Carolina.

"I went through South Carolina and never said a bad word about Senator Obama - not one," Bill Clinton said in an interview with an MTV college station in New Orleans.

Perceptions of racially tinged politics have cost Hillary Clinton support with black voters, who preferred Obama by eight to one in South Carolina and backed him by more than nine to one in a contest in Mississippi last week.

Hillary Clinton's advisers have had little to say about the controversy involving Wright, deflecting questions by saying the issue was something voters would have to keep in mind.