The idea is that legal workers are less likely to be exploited because they would be protected by the law rather than operating outside of it, Avendano said.
But not all union members are convinced that legalizing the estimated 11 million undocumented immigrants in the country is in their best interest.
In 2010, the Center for Immigration Studies, a Washington think tank that opposes illegal immigration and advocates for a reduction in the number of legal immigrants allowed to come to the country, cited a Zogby poll that showed 60 percent of union household supported enforcing current immigration laws. Less than 30 percent supported legalization.
"The right wing has done a really good job of convincing workers that it's against their own interest to join with immigrants," Avendano said.
In the three years since the AFL-CIO began calling for legalization of illegal immigrants, it's been trying to pass its message on to membership.
Some immigrants do drive down wages, said Baldemar Velasquez, president of the Farm Labor Organizing Committee (FLOC), which represents about 10,000 agricultural workers in the Midwest and the South.
"Not because they have a visa or are illegal, but because they're not organized, and don't have a collective bargaining agreements to improve standards," he said.
How many are undocumented is unknown.
The AFL-CIO doesn't ask members their legal status, relying instead on employers and their employees to comply with the law, AFL-CIO spokesman Gonzalo Salvador said.
According to the Pew Hispanic Center, a quarter of farm workers nationwide were undocumented in 2008. FLOC believes that 90 percent of as many as 30,000 laborers working in North Carolina's tobacco fields are undocumented. About 5,000 of them are members of FLOC, Velasquez says.
According to a report released last year by FLOC and Oxfam, an international confederation that fights poverty, conditions on some of the farms were troubling. Some workers reported earning less than minimum wage or that their wages were not enough to meet their basic needs, others said that they were not given access to fresh water in the fields and weren't given breaks to go to the bathroom.
R.J. Reynolds does not grow its own tobacco or employ farm workers.
"Because they are not our employees, we have no direct control over the sourcing of farm laborers, their training, their pay rates, or their housing and access to human services," the company stresses on its website.
Velasquez said that the tobacco producers like Reynolds do have the power to change working conditions in the fields because they buy the tobacco from growers.
In April, the tobacco compamies and FLOC agreed to meet to talk about conditions, according to FLOC.
In 2011, Hispanics made up about 12 percent of union membership that year, according to the Bureau of Labor Statistics. Blacks represented 13 percent of union membership, and Asians made up 4 percent. The rest - about 80 percent - were white.