Back in Iowa, where they export a lot of soybeans, you didn't hear much about NAFTA.

But as the Democratic presidential candidates court votes in Ohio, a state battered by manufacturing job losses, the 15-year-old North American Free Trade Agreement has reached new prominence.

Each of the surviving Democrats has accused the other of distorting their positions on it. Each has accused the other of trying to walk away from previous support.

The heat and contention obscure the reality that Sens. Hillary Rodham Clinton and Barack Obama hold virtually indistinguishable positions -- not to mention the fact that it would be difficult for either to follow through on their boldly stated plans to reform the trade pact.

One of Mrs. Clinton's first campaign stops in Ohio was at the General Motors plan in Lordstown, where employment is down to 3,800 from a peak of more than 10,000. Already on the defensive on the subject of NAFTA, Mrs. Clinton complained about mailings from the Obama campaign that portrayed her as complicit in her husband's championing of it and describing it as a "boon" to U.S. workers.

Mrs. Clinton did praise NAFTA, both while first lady and in her memoirs, but FactCheck.org, a nonpartisan watchdog Web site, found that she never used the word "boon" to describe its effects. That was the paraphrase in a news account of her position.

Her reservations about trade, moreover, are not a campaign trail conversion -- at least not this particular campaign trail. Mrs. Clinton spoke of her reservations as far back as her races for the U.S. Senate.

In reaction to the Obama offensive, the Clinton campaign has papered Buckeye Democrats with mail claiming that the Illinois senator had spoken warmly of NAFTA before a group of Illinois farmers.

The Obama campaign, in an assertion backed up by FactCheck, said that the flier's quotes were taken out of context and had omitted skeptical comments on the effects of trade. The campaign also noted that the Chicago Tribune, while endorsing his 2004 Senate candidacy, took note of the fact that they disagreed with Mr. Obama's anti-NAFTA stance.

Mr. Obama faults the New York senator for her judgment in the past -- when she praised the accord after its 1993 ratification, but Tuesday's debate demonstrated that their plans for the future sound the same.

In response to a question from NBC's Tim Russert, on whether she would exercise the U.S. right to withdraw from the pact, she said, "No. I will say, we will opt out of NAFTA unless we renegotiate it and we renegotiate it on terms that are favorable to all of America.''

To the same question, Mr. Obama made clear he had the same answer: "I will make sure that we renegotiate in the same way that Sen. Clinton talked about, and I think actually Sen. Clinton's answer on this one is right. I think we should use the hammer of a potential opt-out as leverage to ensure that we actually get labor and environmental standards that are enforced."

An aide to Mrs. Clinton said that the senator wanted to see stronger labor and environmental standards within the core agreement of the treaty. At the time the pact was adopted, those standards, criticized by organized labor and others as too weak, were included only in side agreements. She also would mandate staffing increases for the office of the U.S. Trade Representative to provide more manpower to enforce existing trade rules. More technically, Mrs. Clinton would also like to reform NAFTA provisions that allow foreign corporations to challenge U.S. regulations.

Mr. Obama has offered the broad goal of strengthening labor, environmental and safety standards of NAFTA, but his campaign did not respond to requests for more specific examples of how he would amend the pact, and among the extensive issue statements on the candidate's Web page, there are no details on how exactly he would like to change it.

While both Democrats have said they favor trade in general and recognize the realities of an increasingly globalized economy, their debate statements Tuesday irked at least some trading partners. The Toronto Star reported that the Canadian Trade Minister said that a NAFTA revision could entail costs as well as benefits to the United States, and that those costs could include a loss of open U.S. access to Canada's energy.

"Knowledgeable observers would have to take note of the fact that we are the largest supplier of energy to the U.S. and NAFTA has been the foundation for integrating the North American energy market,'' David Emerson said in the Star report. "When people get below the rhetoric and pick away at the details, they are going to find it's not such a slam dunk proposition.''

Canada's chief diplomat said, diplomatically, that the Democrats might not know what they were talking about.

"I realize they are in the middle of a presidential nomination race, so they have many things on their minds," said Foreign Minister Jim Flaherty. "Perhaps the nominees have not had a chance to familiarize themselves with the benefits to America and the American economy of NAFTA because there is a tendency to say it favors Mexico or it favors Canada rather than to recognize the mutual benefits that come out of free trade.''

CTV, the Canadian television network, reported that Mr. Obama's campaign had alerted Canadian diplomats that they should expect and ignore anti-NAFTA rhetoric on the campaign trail.

The embassy in Washington issued a denial that any such conversation had taken place, as did the Obama campaign.

"When Senator Obama says that he will forcefully act to make NAFTA a better deal for American workers, he means it," said Bill Burton, Mr. Obama's press secretary. "Both Canada and Mexico should know that, as president, Barack Obama will do what it takes to create and protect American jobs and strengthen the American economy -- that includes amending NAFTA to include labor and environmental standards."

The Canadian officials are not the first foreign diplomats critical of the rumblings on the Democratic primary trail. Peter Mandelson, the chief trade official of the European Union, decried the tone of the Democratic race in an interview with The Wall Street Journal.

"Most of the Democrat free traders seem to have taken to the hills," Mr. Mandelson lamented. "I wish there was more push back within the Democrat Party because they know this is right."

Sen. John McCain, the apparent GOP nominee, is an unapologetic free-trader, although his trailing rival, former Arkansas Gov. Mike Huckabee has complained of the effects of trade deals on American workers.

"I want to tell our Canadian friends, I want to tell our friends in Mexico and other trading partners around the world that I will negotiate and conclude free trade agreements and I will not, I will not, after entering into solemn agreements, go and say that I will abrogate those agreements," Mr. McCain said in Texas Friday, according to the Web site Politico.com.

Alan Deardorff, associate dean of the Gerald R. Ford School of Public Policy at the University of Michigan, said the scholarly consensus about NAFTA is that its overall effect has been positive but modest -- a view that runs contrary to its critics and to the lofty promises that helped bring its passage.

Like all recent trade agreements, NAFTA was negotiated under so-called "fast-track authority,'' which requires Congress to vote up or down on trade agreements and bars amendments by the legislators. The theory behind the authority is that foreign governments would have a hard time reaching agreements with the executive branch if they faced the uncertainty of further tinkering by Congress.

The president's fast-track authority expired last July, and until and unless it is renewed, it would be difficult for any new trade treaties -- or a rewritten NAFTA -- to make their way from negotiation through ratification.

"To me, it looks insane to think you could negotiate any kind of trade deal with the understanding that once we take it to Congress, Congress could change it," said Mr. Deardorff.

Claude Barfield, a resident scholar at the American Enterprise Institute, saw some irony in the trade statements from Democrats.

"Here they've been claiming that the U.S. is going around [the world] acting like a bully and then they turn around and say to Canada and Mexico, 'Here's what you have to change.' "

Of course, a decision to opt out of NAFTA without an amended or successor agreement would be just fine with some Democratic constituencies, who see the pact as a servant of corporate interests. Neither Mr. Obama nor Mrs. Clinton say they want to abandon either NAFTA, or free trade, but negotiating and seeking approval for an altered NAFTA would be a daunting challenge.

NAFTA passed the U.S. House by the relatively narrow margin of 234 to 200 with a majority of Democrats in the minority. And that was at a time when the voices against trade were considerably less well-organized and active.

"The Democratic Party has more anti-trade, anti-globalism activism now than in 1992," said Mr. Barfield, the AEI trade expert. Of the pledges last week to renegotiate NAFTA early in a new Democratic administration, he said, "They've really painted themselves into a corner; I don't see how they get out now."