Canada and Mexico are rebuking the top Democratic presidential candidates, senators Clinton and Obama, for vowing to renegotiate the North American Free Trade Agreement and threatening to pull out of the pact altogether if America’s neighbors refuse.

“Nafta has been a win-win-win for Mexico, the United States and Canada, proving that a rising tide can lift all boats,” the Mexican ambassador to America, Arturo Sarukhan, said in a written statement. “Mexico does not support reopening Nafta. It would be like throwing a monkey wrench into the engine of North American competitiveness.”

Canada’s finance minister, James Flaherty, said he was troubled that the candidates’ anti-Nafta rhetoric was departing from reality. “It is a concern,” he told reporters in Toronto, according to the Canadian Broadcasting Corp. “I realize they are in the middle of a presidential nomination race so they have many things on their mind. But I would think that it’s very important that whoever the nominee is enters into a discussion with those who are very knowledgeable about Nafta.”

“Nafta is of tremendous benefit to Americans,” Mr. Flaherty said.

Canada’s trade minister, David Emerson, also lamented the threats of withdrawal. “The rhetoric of protectionism has been creeping up and getting more strident,” Mr. Emerson said in Ottawa, according to Bloomberg News. “It’s not just the heat of the presidential campaign.”

Mr. Emerson suggested that America might regret choosing to put the entire trade agreement up for new negotiations. “If you reopen Nafta, you re-open it for all three parties,” he said. “If Nafta were reopened, we would have our list of priorities.”

The trade official even made what some Canadian journalists interpreted as a veiled threat that new Nafta talks could lead to a reduction in Canada’s natural gas and oil exports to America. “Knowledgeable observers would have to take note of the fact that we are the largest supplier of energy to the U.S.,” Mr. Emerson said, according to the Canadian Press. “It’s not such a slam dunk proposition.”

A diplomatic source, who asked not to be named, said the statements signaled considerable pique, as foreign governments usually take pains to refrain from any public commentary on American political campaigns.

At a debate Tuesday in Ohio, Mrs. Clinton and Mr. Obama were asked if they would respond to Canadian and Mexican unwillingness to reopen the trade deal by exercising America’s right to withdraw with six months’ notice.

“No. I will say we will opt out of Nafta unless we renegotiate it and we renegotiate on terms that are favorable to all of America,” Mrs. Clinton said.

“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. And that is not what has been happening so far,” Mr. Obama said.

The Democratic candidates seemed eager not to be outdone in expressing their hostility to a trade deal that is deeply unpopular in Ohio, Illinois, and other rust belt states that have lost millions of manufacturing jobs in recent years. However, analysts of trade among America, Canada, and Mexico disputed the notion that Nafta was responsible for the economic pain in those Midwest industrial states.

“The problems Ohio is having, whatever they may be, have nothing to do with Nafta. They have to do with the fact that the world is changing,” a professor of economics at Carleton University in Ottawa who helped negotiate the deal, Michael Hart, said. “Trade agreements after 15 years have been so fully implemented that the idea you’re going to undo it is a ridiculous idea. They’re going to reimpose tariffs?”

Mr. Hart said there is no outrage in Canada over the Nafta pact. “We’re past that,” he said. “We’re a trade-dependent country.”

Even a Canadian Nafta critic, Gordon Laxer of the University of Alberta, said there is little animus toward the deal. His main objection is to provisions that require Canada to maintain oil and gas flows to America. “We’re more and more dependent on Opec,” he complained.

A Mexcian Nafta specialist said Mexico may have taken some jobs from Ohio 10 or more years ago, but any recent job migration has been to India or China, which offer even lower wages. “It would be very nice if all those businesses came to Mexico, but that’s not the case,” Jose Romero of the College of Mexico said. “We got some benefit at the beginning. We were the only underdeveloped country to receive special treatment from the U.S., but after that the U.S. signed treaties with many other countries.”

Senator Grassley of Iowa, a Republican, said yesterday that America would be the loser if it pulled out of Nafta. “I don’t understand what Senator Clinton and Senator Obama hope to gain,” Mr. Grassley said. “Nafta is often used as a scapegoat. But without it, Mexico would still benefit from much lower tariffs on exports to the United States than vice versa.”