RALEIGH — Democratic presidential prospect John Edwards is throwing his weight behind labor activists who are trying to pressure the world’s largest hog slaughterhouse into recognizing a union without an employee vote.

Edwards, in a letter to Smithfield Foods CEO Larry Pope, urged the company to remain neutral during an organizing campaign at the Smithfield Packing Co. plant in Tar Heel. Smithfield executives have said they would support a secret-ballot vote, but union leaders argue the company won’t allow a fair election.

“I hope and expect that you will protect the right of your workers in North Carolina and across the country to form a union and bargain collectively,” Edwards wrote in the letter dated Monday and released by his campaign Wednesday.

The former North Carolina senator has become a leading advocate for unions and labor groups. For two years after his failed bid for the White House in 2004, Edwards traveled the nation, joining picket lines with the likes of Teamsters President James Hoffa and pushing for a higher minimum wage.

He also joined in labor’s call for card-check neutrality, which would allow labor groups to certify their standing as soon as they get the majority of workers to sign cards.

But Smithfield spokesman Dennis Pittman questioned why Edwards would support card-check and not a vote among employees.

“I bet if (Edwards) had to be elected by card-check, he’d have a fit,” Pittman said.

For more than a decade, activists allied with the United Food and Commercial Workers International union have been trying to organize the Smithfield plant about 80 miles south of Raleigh.

The facility employs roughly 5,500 workers and slaughters up to 32,000 hogs daily, and it has become a focal point for labor leaders trying to gain traction in Southern manufacturing industries.

Smithfield officials have said they would support a secret-ballot election to let employees decide whether to organize, but union activists have scoffed at the offer, citing a history of meddling.

A federal appeals court found that Smithfield improperly influenced two union elections held in 1993 and 1997 during which employees voted against organizing. In a settlement reached earlier this year, the company agreed to pay $1.1 million in back wages, plus interest, to workers fired as part of the dispute.

For that reason, Leila McDowell, a spokeswoman for the union-affiliated Smithfield Justice Campaign, said that organizers would not consider another secret-ballot election.

“You can’t have free and fair elections at a plant like Smithfield, not in the past, not in the future,” McDowell said, adding that union leaders prefer alternative organizing methods, such as card-check.

But businesses have complained that card-check takes away the employee’s right to vote and gives labor organizers the chance to pressure workers to join their ranks.

“There’s really no regulation over how the cards could be obtained,” said Pittman, the Smithfield spokesman. “Employees could be pressured. They could be badgered.”

Over the past year, the union has accelerated its efforts to pressure Smithfield executives. Last weekend, union activists picketed at 24 Harris Teeter locations in several states, urging the North Carolina-based grocer to remove Smithfield products from its shelves.

Harris Teeter has declined to dump Smithfield products, saying the long-standing organizing stalemate needs to be resolved between Smithfield and the union.