US:BARACK OBAMA and Hillary Clinton have exchanged sharp jibes at what could be the last debate of the Democratic primary campaign, as Mr Obama faced tough questions about associations that could prove embarrassing in a general election.
The debate at Philadelphia's National Constitutional Centre, which was sponsored by ABC News and Pennsylvania's WPVI-TV, was the 21st for the two candidates since the presidential campaign began last year.
The first 50 minutes were dominated by questions about recent controversies over Mr Obama's former pastor Jeremiah Wright, his remarks about small-town Americans and his association with a former urban guerrilla, as well as Mrs Clinton erroneous claim that she dodged sniper fire during a visit to Bosnia in the 1990s.
There was little disagreement between the candidates on policy issues, but the debate left voters with a stark choice between Mr Obama's vision of a new kind of politics and Mrs Clinton's claim that she is the Democrat best placed to win in November in the current political climate.
Mr Obama appeared taken aback by the relentless questioning about his description of rural Americans as bitter, and his controversial friends and his exasperation broke through a number of times. "The problem that we have in our politics, which is fairly typical, is that you take one person's statement, if it's not properly phrased, and you just beat it to death. And that's what Senator Clinton's been doing over the last four days," he said. "And I understand that. That's politics. And I expect to have to go through this process. But I do think it's important to recognise that it's not helping that person who's sitting at the kitchen table who is trying to figure out how to pay the bills at the end of the month."
He said his candidacy offered the US an opportunity to move beyond the racial divisions that inspired the controversial statements of his former pastor, who said African-Americans should not say "God Bless America" but "God Damn America".
Mrs Clinton repeated, however, that she would have left Rev Wright's church after the pastor described the terrorist attacks of 9/11 as America's chickens coming home to roost. "You get to choose your pastor. You don't choose your family, but you get to choose your pastor," she said.
Mr Obama was also pressed on his association with Bill Ayers, a Chicago professor of English who was a member of the 1960s revolutionary organisation, the Weather Underground, also known as the Weathermen. An offshoot of the Maoist Revolutionary Youth Movement within Students for a Democratic Society, the group was responsible for bomb attacks on the Pentagon, the Capitol and a New York police station in the early 1970s, although the only deaths it caused were of its own members in an accidental explosion in Greenwich Village.
Mr Ayers, who was indicted in 1970 for inciting to riot and conspiracy to bomb government buildings, saw all charges dropped in 1974 because of prosecutorial misconduct. He hosted a fundraising event for Mr Obama in Chicago and the two men served together on a charitable board, but the Illinois senator said in Philadelphia that they seldom exchanged ideas.
"So this kind of game in which anybody who I know, regardless of how flimsy the relationship is, that somehow their ideas could be attributed to me, I think the American people are smarter than that. They're not going to suggest somehow that that is reflective of my views, because it obviously isn't," Mr Obama said.
Mrs Clinton said that, regardless of nuances, Republicans were certain to target the Democratic candidate in the general election over every detail of their past associations, and it was crucial that the Democrat should be able to withstand such attacks. "You know, I've been in this arena for a long time. I have a lot of baggage, and everybody has rummaged through it for years," she said.
"And so, therefore, I have an opportunity to come to this campaign with a very strong conviction and feeling that I will be able to withstand whatever the Republicans send our way." She said, however, that she believed Mr Obama could defeat Republican John McCain if he becomes the Democratic nominee and promised to work tirelessly on his behalf if she loses her party's contest.
Mr Obama's campaign complained after the debate about the focus on what they dismissed as "process issues" rather than policy and chief strategist David Axelrod said that Pennsylvania represented a tough challenge for the candidate, despite outspending Mrs Clinton in the state by a margin of at least two to one.
Mrs Clinton's communications chief Howard Wolfson expressed delight at her performance and defended as legitimate the focus on controversial issues. "I think it was really a preview for both sides of what we would see if either one of them is the nominee," he said.