MILWAUKEE — On May 27, about 150 workers from Palermo’s Pizza factory here, representing three-fourths of its production workers, met to sign a petition saying they wanted to unionize. They say they gave the petition to management two days later.


Around the same time, Palermo’s delivered letters to 89 immigrant workers, asking them to provide documentation verifying that they had the right to work in the United States. Ten days later, almost all of them were fired.


Labor organizers assert that Palermo’s, one of the nation’s largest producers of frozen pizza, was trying to snuff out a unionization drive in its infancy. The company says it was merely responding to warnings it had received from federal immigration authorities to fire unauthorized workers or face hefty fines.


Scores of Palermo’s workers have been on strike since June 1 to protest this immigration crackdown, as well as what they say were poor wages and working conditions. Day after day, the strikers picket outside the factory, often in 90-degree heat, chanting, “No justice, no pizza.” Labor unions across the nation have rallied behind them and called for a boycott of Palermo’s products.


At a time when labor leaders see immigrants as a group ripe for unionization, the conflict highlights how difficult it can be to organize workplaces that include unauthorized workers, who are entitled to certain labor protections despite working illegally.


The fight also demonstrates how the Obama administration’s campaign to toughen immigration enforcement in workplaces can increase employers’ leverage to derail unionization efforts.


After several labor leaders complained that the enforcement action at Palermo’s was undermining a unionization effort, United States Immigration and Customs Enforcement announced on June 7 that it was staying the enforcement, the first time it has ever suspended a crackdown that way, according to immigration experts. Although the agency’s move might have been too late to help the fired Palermo’s workers, labor experts say the government’s change of heart might affect future efforts to unionize immigrants.


“There has been a history of the federal government not understanding how its enforcement can undermine union organizing drives,” said Janice Fine, a labor relations professor at Rutgers. “There is no question that this is a new moment.”


Amid strong public support for more immigration enforcement, the Obama administration has stepped up pressure on employers to dismiss or avoid hiring illegal immigrants. During President Obama’s tenure, federal inspectors have audited 8,079 companies suspected of hiring unauthorized workers, leading to tens of thousands of immigrant workers quitting or being fired.


Labor leaders say companies often taken advantage of workers’ illegal status to violate wage and safety laws or otherwise exploit them. When immigrant workers band together to protest or seek to unionize, union leaders say, companies sometimes invite in immigration officials to deliberately undercut them.


Unions cite enforcement actions at Smithfield’s huge pork processing plant in Tar Heel, N.C., and the Agriprocessors meatpacking plant in Postville, Iowa, as examples, although both employers denied that they had asked immigration enforcers to intervene.


Still, unions have had some success organizing industries that employ many illegal immigrants, like janitorial companies, poultry plants and carwashes.


There is some irony in the Palermo’s dispute — a company that boasts that it was founded by immigrants finds itself in battle with its immigrant employees.


Palermo’s executives say their company is an inspiring, up-by-the-bootstraps story: Gaspare and Zina Fallucca, husband and wife immigrants from Sicily, founded a small Italian bakery on Milwaukee’s east side in 1964 that has since blossomed into a company that produces millions of pizzas yearly.


But dozens of Latino immigrants employed by the company assert that the couple’s descendants, who now run the company, paid and treated them so poorly that it prevented them from realizing their own American dream.