The protectionist tone of the Democratic presidential race is reaching new heights as senators Clinton and Obama vowed last night to withdraw America from the North American Free Trade Agreement if Mexico and Canada do not agree to renegotiate the terms of the pact.

Both senators publicly threatened such a pullout during a televised debate in Ohio, a state that is smarting from a general economic slowdown and the loss of hundreds of thousands of manufacturing jobs.

“I will say we will opt out of NAFTA unless we renegotiate it,” Mrs. Clinton said in response to a question about whether she would take advantage of a provision in the deal that allows withdrawal from the pact on six months’ notice.

“I will make sure that we renegotiate in the same way Senator Clinton talked about,” Mr. Obama said. “I think we should use the hammer of an opt-out to make sure that we actually get labor and environmental standards that are enforced.”

Last night’s 90-minute debate, held at Cleveland State University, was more contentious and tense than the last exchange between the top Democrats. It was also the last scheduled debate between Mrs. Clinton and Mr. Obama before the primaries next Tuesday in Ohio, Rhode Island, Texas, and Vermont. The stakes were particularly high for Mrs. Clinton, who has been slipping in opinion polls and who badly needs victories in Ohio and Texas to remain a viable contender for the nomination.

Mrs. Clinton’s campaign, which has clashed on several occasions with journalists from NBC News, had expressed reservations about taking part in the NBC-sponsored debate. Less than 20 minutes into the exchange yesterday, Mrs. Clinton complained that she had received an undue number of “first questions” during the debates. She said the queries were emblematic of what some perceive as the press corps’s pro-Obama coverage — a phenomenon that is now the subject of popular satire. “If anybody saw ‘Saturday Night Live,’ maybe we should ask Barack if he’s comfortable and needs another pillow,” she said.

One query that irritated her was about the posting online of a photograph of Mr. Obama in a traditional Somali costume — including a turban — during a trip to Africa. Asked if she could say flatly that her campaign did not send out the photo, Mrs. Clinton said, “So far as I know, it did not, and I certainly know nothing about it and have made clear that’s not the kind of behavior I condone and expect from anyone working in my campaign.” She noted that she had dismissed campaign aides involved in improper tactics.

Mrs. Clinton’s stance was at odds with her campaign’s initial reaction when the photo surfaced on Monday. At that time, her campaign manager, Margaret Williams, issued a statement arguing that the image was not objectionable and that Mr. Obama’s campaign was being “divisive” by charging that the episode amounted to dirty tactics.

Mr. Obama said he took Mrs. Clinton at her word but that he had been repeatedly subjected to negative and inaccurate mailers and phone calls from her camp. “Senator Clinton, at least her campaign, has constantly sent out negative attacks on us,” he said. “We haven’t whined about it.”

Mrs. Clinton insisted that some of Mr. Obama’s mailings went too far by mimicking the so-called Harry and Louise ad campaign that torpedoed her health care reform efforts in the 1990s. “It is almost as though the health insurance companies and the Republicans wrote it,” she said.

Mrs. Clinton also argued that Mr. Obama’s plan not to mandate health insurance coverage was akin to suggesting that people should be able to opt in or out of Social Security and similar programs, a notion anathema to most Democrats.

“In fact, Medicare Part B is not mandated, it is voluntary, and yet people over 65 choose to purchase it, Hillary, and the reason they choose to purchase it is because it’s a good deal,” Mr. Obama said. Though many experts contend it will be costly and difficult to drive costs down without a mandate that everyone buy or acquire insurance, Mr. Obama insisted his plan would lower costs even more than Mrs. Clinton’s.

At one point, Mr. Obama was asked if he rejected the support he has recently received from a Nation of Islam leader who has demonized Jews, Minister Louis Farrakhan. “I have been very clear in my denunciation of Minister Farrakhan’s anti-Semitic comments,” the senator said. “I obviously can’t censor him, but it’s not support that I sought.”

Mr. Obama added that he was ” a stalwart friend of Israel” and that the Jewish state’s security was “sacrosanct.”

Sensing an opening, Mrs. Clinton jumped in to describe her Senate race in 2000, when she received the support of the Independence Party, whose leader, Lenora Fulani, had made comments critical of Jews and Israel. “I made clear that I did not want their support. I rejected it,” Mrs. Clinton said, adding that she risked losing support among voters but took “a principled stand.”

Pressed on whether she was suggesting Mr. Obama did not go far enough in distancing himself from Mr. Farrakhan, Mrs. Clinton replied, “No. I’m just saying you asked specifically if he would reject it, and there’s a difference between denouncing and rejecting.”

“I have to say, I don’t see the difference between denouncing and rejecting,” Mr. Obama responded. “There was no formal offer of help from Minister Farrakhan that would involve me rejecting. But if reject, Senator Clinton feels, is stronger than the word, denounce, then I’m happy to concede the point. And I would reject and denounce.”

Mrs. Clinton passed up another opportunity to call Mr. Obama unqualified to be commander in chief. She praised his anti-war speech from 2002 but went on to minimize its significance. “Many people gave speeches against the war then,” she said. “He didn’t have responsibility. He didn’t have to vote.”

“My objections to the war in Iraq were not simply a speech,” Mr. Obama replied. “I was in the midst of a U.S. Senate campaign.” The Illinois senator said Mrs. Clinton’s claim that the two now have identical Senate voting records on Iraq was beside the point. “Once we had driven the bus into the ditch, there were only so many ways we could get out. The question is who is making the decision initially to drive the bus into the ditch,” he said, adding that Mrs. Clinton had “facilitated and enabled” President Bush to make a decision that damaged national security.

Mrs. Clinton, who has steadfastly refused to apologize for her vote authorizing the war, said for the first time last night that she would like to have that vote back.