Immigration raids that purged the world's largest pork slaughterhouse of illegal Hispanic workers may have made it ripe for unionization.

The News & Observer of Raleigh reports that the approximately 5,000 workers who remained at Smithfield Foods' Tar Heel plant after immigration raids were mostly black, whom experts say are traditionally pro-union.

Workers voted over two days last month to approve unionization for the hog processing plant.

Federal immigration agents arrested illegal workers in the plant in January 2007 and again that August. The company also began firing workers whose Social Security numbers didn't match federal records.

Lidia Victoria, a 13-year employee and a U.S citizen born in the Dominican Republic, said she tried for years to convince Latino workers of the union's benefits.

Mattie Fulcher, another worker, said union supporters worked hard to build unity between the plant's black and Hispanic workers. And she said that, at least before the raids, many Hispanics did become vocal supporters of the union.

But, Victoria said, the raids and firings left many - even those in the country legally - too frightened to speak up for unionization.

"At the end, they turned their back to the union," Victoria said this week. "The union won because black people went to vote yes."

The company says the share of Hispanic workers shrunk to about a quarter, down from about half in early 2007. Blacks now make up 54 percent of the work force.

Blacks are traditionally more supportive of unions that other races and ethnic groups, labor experts have said, suggesting the shift to a majority black work force could have secured the union victory.

"With this real crackdown on immigration that we've seen, it's been very intimidating," said Richard Hurd, a professor of industrial and labor relations at Cornell University in New York. "There were a lot of things at play in the situation at Smithfield, and that's one aspect of it."

Marion Crain, a labor law expert at Washington University in St. Louis, Mo., said African-Americans' strong support of unions stems from a sense of racial unity, stretching back to slavery days.

But company and union officials said they doubt the racial makeup influenced the election's outcome.

Dennis Pittman, the communications director for Smithfield Foods' 13 packing plants, said the work force was majority black during a previous union election in 1997.

The results of that election were thrown out after a judge ruled union supporters were fired by the company.