Clinton said Obama's campaign had recently sent out mass mailings with false information about her health care proposal, adding, "it is almost as though the health insurance companies and the Republicans wrote it."
When it was his turn to speak, Obama said Clinton's campaign has "constantly sent out negative attacks on us ... We haven't whined about it because I understand that's the nature of these campaigns."
The tone was polite yet pointed, increasingly so as the 90-minute session wore on, a reflection of the stakes in a race in which Obama has won 11 straight primaries and caucuses and Clinton is in desperate need of a comeback.
Clinton also said as far as she knew her campaign had nothing to do with circulating a photograph of Obama wearing a white turban and a wraparound white robe presented to him by elders in Wajir, in northeastern Kenya.
"I take Senator Clinton at her word that she knew nothing about the photo," Obama said.
In one curious moment, Clinton said, "In the last several debates I seem to get the first question all the time. I don't mind. I'll be happy to field it. I just find it curious if anybody saw "Saturday Night Live," maybe we should ask Barack if he's comfortable and needs another pillow."
In its episode last Saturday, the comedy show ran a feature portraying the news media as going easy on Obama, and a questioner asking at one point if he was comfortable and needed another pillow.
The two rivals, the only survivors of a grueling primary season, sat about a foot apart at a table on stage at Cleveland State University. It was the 20th debate of the campaign, 10 months to the day after the first.
The race was far different in April 2007, Clinton the front-runner by far. Now Obama holds that place, both in terms of contests and delegates won. The two square off next Tuesday in primaries in Ohio, Texas, Rhode Island and Vermont, with 370 delegates at stake.
Both Obama and Clinton were on the receiving end of pointed questions from Tim Russert of NBC News, one of two moderators for the event.
Asked whether he was waffling on his pledge of agreeing to take federal funds for the fall campaign, Obama said he was still contesting the primaries.
"If I am the nominee I will sit down with John McCain and make sure we come up with a system that is fair to both sides," he said. Obama could presumably raise far more money than the federal system provides, but accepting government money precludes that.
The equivalent question to Clinton concerned the income tax returns that she and her husband, former President Clinton, file jointly.
"I will release my tax returns," Clinton said, if she becomes the Democratic nominee. She then added she might do so "even earlier," but not before Tuesday's primary.
The two rivals also debated NAFTA, the free trade agreement with Canada and Mexico that is wildly unpopular with blue-collar workers whose votes are critical in any Democratic primary in Ohio.
Neither one said they were ready to withdraw from the agreement, although both said they would use the threat of withdrawal to pressure Mexico to make changes.
"I have said I would renegotiate NAFTA," said Clinton. "I will say to Mexico that we will opt out of NAFTA unless we renegotiate it."
Obama said Clinton has tried to have it both ways, touting the trade deal in farm states where it's popular while finding fault with it in places like Ohio.
"This is something I have been consistent about," said Obama, who said he went to the American Farm Bureau Federation to tout his opposition and used it as an issue in his 2004 Senate campaign.
"That conversation I had with the Farm Bureau, I was not ambivalent at all," said Obama.
On the war, both candidates denounced President Bush's record on Iraq, then restated long-held disagreements over which of them was more opposed.
Clinton said she and Obama had virtually identical voting records on the war since he came to the Senate in 2005.
The former first lady voted in 2002 to authorize the war, at a time when Obama was not yet in Congress. Asked whether she'd like to have the vote back, she said, "Absolutely. I've said that many times."
Obama tried to use the issue to rebut charges that he is ill-prepared to become commander in chief.
"The fact is that Senator Clinton often says that she is ready on day one, but, in fact, she was ready to give in to George Bush on day one on this critical issue," Obama said.
Clinton also stumbled at one point as she tried to pronounce the name of Dmitry Medvedev, Russia's first deputy prime minister, who is expected to win an election to succeed President Vladimir Putin on Sunday. "Whatever," she said after several attempts to demonstrate she knew his name.
Obama also sought to distance himself from an endorsement from Nation of Islam Minister Louis Farrakhan, the controversial Chicago-based minister who has made numerous anti-Semitic comments in the past.
Obama said he hadn't sought the endorsement, and that he had denounced the remarks.
Clinton interjected at one point, saying that in her initial Senate campaign in New York in 2000, she was supported by a group with virulent anti-Semitic views.
"I rejected it, and said it would not be anything I would be comfortable with," she said. Clinton said rejecting support was different from denouncing it, an obvious jab at Obama.
He responded by saying he didn't see the difference, since Farrakhan hadn't done anything except declare his support. But given Clinton's comments, he said, "I happily concede the point and I would reject and denounce."
The audience applauded at that.
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Associated Press writers Mike Glover and Tom Raum in Cleveland
and M.R. Kropko in Lorain contributed to this report.
