The free-trade agreement with Canada barely caused a ripple for American politicians. It was hailed as a historic breakthrough by the Reagan administration and cleared Congress with hardly a dissenting word.

It is a different story with Mexico. With a vast disparity in economic and social conditions between the United States and Mexico, opposition formed even before President Bush formally sought permission from Congress to negotiate a North American free-trade agreement. Even congressional supporters raised questions over the desirability of forging close commercial ties between countries with such different degrees of economic power as the United States, Canada and Mexico.

With the U.S. economy 10 times greater than Canada's, the political ripples over the U.S.-Canada free-trade agreement were generally north of the border, where nationalistic-minded Canadians feared their economic independence would be lost as a result of closer ties with the United States.

The United States is even more of a giant when compared to Mexico, but the usually vocal Mexican left has been muted -- at least so far -- in the debate.

By contrast, a wide variety of U.S. political, economic and social interests, headed by organized labor, are questioning the validity of expanding the U.S.-Canadian free-trade agreement to include Mexico.

As a tactical step, the U.S. opponents are focusing their attack on giving the Bush administration "fast track" authority, which allows it to bring up an agreement to Congress within two years with no amendments nor delaying tactics. Their real concerns, however, have more to do with fears of losing jobs and factories to Mexico.

The administration has been downplaying the vast economic differences between the United States and Mexico. Bush trumpeted a North American free-trade zone, saying it would create the world's largest open market consisting of 360 million people with a combined output of $6 trillion. He failed to mention, however, that almost all of the economic strength and a vast majority of the people come from the United States, a sharp difference from the European Community's common market, where the economic disparity among its 12 members is far less.

The U.S. economy is the largest in the world, totaling some $5.2 trillion, and it has a population 10 times greater than Canada's and almost three times larger than Mexico's. As a result, U.S. Trade Representative Carla Hills said, "I can't claim massive winning sectors {for U.S. industry} because the Mexican economy is so small."

"Never has a developed country of our wealth and size tried to negotiate a free-trade agreement with a developing country like Mexico. Mexican wage rates are one-seventh U.S. levels. That concerns many U.S. industries and workers. It concerns me," Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) said Wednesday, at the opening congressional hearing into the agreement. Bentsen is one of the agreement's strongest congressional supporters.

Organized labor fears low wages will lead to a flight of U.S. jobs to Mexico as companies move south with the certainty that a free-trade pact will allow them to send products into the United States without tariffs.

"The AFL-CIO believes that the huge differences that presently exist between the United States and Mexico make the establishment of a free-trade area both damaging to U.S. workers and of little benefit to Mexican workers," said Thomas R. Donahue, secretary-treasurer of the AFL-CIO.

"What we need," said Craig Merrilees of the National Toxics Campaign, "is a trade agreement that will promote social and environmental justice on both sides of the border."

Like organized labor, environmentalists fear that U.S. businesses will skip to Mexico to avoid stricter U.S. pollution-control laws.

Hills gave short shrift to the environmental arguments, saying Mexico has environmental laws similar to those in the United States, even though enforcement sometimes lag, and that U.S. companies are unlikely to flee to Mexico because of the pollution laws.

The big support for the Mexico accord comes from major business groups such as the Business Roundtable and the U.S. Chamber of Commerce, which said an agreement would help U.S. competitiveness.

But U.S. and Mexican officials have said they need to gain support of small and medium-sized businesses in the United States if they are to overcome the fast-developing and highly vocal opposition.

Textile and apparel manufacturers, for example, quickly declared their opposition even though Hills said their sector is running a trade surplus with Mexico.

Mexican Trade Minister Jaime Serra Puche told Washington Post reporters and editors Tuesday that textile manufacturers could be big winners because the fabric and yarn they make with machines in this country can be sold in Mexico, where low-wage workers will do the labor-intensive jobs of turning the cloth into garments.

Regional splits over the free-trade pact also surfaced during the Senate Finance Committee hearing.

Arizona Sens. Dennis DeConcini (D) and John McCain (R), whose state is expected to gain from free trade with Mexico, testified in favor of the agreement.

But Sen. Donald W. Riegle Jr. (D-Mich.), whose largely industrial state might lose high-paying manufacturing jobs, called for any agreement to address labor and environmental standards, which he said "are not enforced in Mexico."