MEXICO CITY – U.S. lobbyists lashed out Wednesday at the Mexican “Nothing Gringo” campaign timed for May 1 to coincide with the “Day Without Immigrants” boycott in the United States.

The American Chamber of Commerce in Mexico said organizers are risking a backlash and foolishly targeting some of their best allies, since U.S. corporations have actively lobbied the U.S. Congress for immigration reform including legalization for many of the estimated 11 million undocumented migrants.

Mexicans’ refusal to “buy American” on May 1 could further polarize the debate and make reform supporters seem anti-American at the very moment that lobbyists are trying to persuade lawmakers in Washington to pass a bill that would benefit migrants, said Larry Rubin, the chamber’s president.

“This is like shooting oneself in the foot,” Rubin said. “U.S. companies have been the first to lobby, launching a huge lobbying effort for immigration reform. … Why hurt something that is helping you?”

Migrants and their supporters in the United States are being encouraged to skip work and school and not spend money for one day to demonstrate the migrants’ importance to the U.S. economy.

South of the border, Mexicans are targeting American stores and chain restaurants – “That means no Dunkin’ Donuts, no McDonald’s, Burger King, Starbucks, Sears, Krispy Kreme or Wal-Mart,” reads one e-mail making the rounds.

But even activists are confused about which companies are U.S.-owned. Sears is cited by boycott organizers, although Sears’ Mexico stores were bought by Mexican billionaire Carlos Slim in 1997. And few organizers mention Vips – the chain of ubiquitous Mexican diners – although they are owned by Wal-Mart Stores Inc.

A quarter of Mexico’s formal private-sector jobs with regular pay are provided by U.S. firms, according to the chamber, including Walmex, the Mexican Wal-Mart subsidiary that is the nation’s biggest private employer with 140,000 workers.

Delphi Corp., the U.S. auto parts maker, is second with 70,000 workers.

“Certainly, companies could be hurt,” Rubin said at a news conference Wednesday.

The chamber represents more than 2,000 American and other foreign companies doing business in Mexico, and members said they are responsible for $100 billion of investment in the country.

The companies’s representatives said they’re helping Mexico by providing jobs, but activists counter they pay so little that Mexicans have little choice but to head north.

Backers of the Mexican boycott insisted Wednesday that the protest could send a message that American companies should offer better pay and benefits to their Mexican workers.

“They continue to exploit Mexicans with badly paid jobs and no labor rights,” said Roberto Vigil, who works in the Mexico City office of the California-based immigrants rights group Hermandad Mexicana. “They’re kind of two-faced: They support, but they exploit.”

Unskilled workers at U.S. companies usually start with Mexico’s minimum wage of $4.35 a day. Although many earn more, such as seamstresses making an average of $5.89 a day – even these wages pale in comparison to paychecks offered by the same companies north of the border, conceded the chamber’s Humberto Banuelos.

A cashier at Subway – or “sandwich artist,” as the company refers to them – earns about $189 a month in Mexico City. In Colorado, Subway cashiers make four times that – $824.

Companies also often hire workers for three-month periods to avoid paying health insurance or other benefits, activists say.

“Yes, we are aware that they are the largest employers in the Mexican republic, but they are paying crumbs,” said Martha Suarez Cantu, coordinator of Alianza Braceroproa, a Mexican labor-rights group helping organize the boycott.