Sens. Chuck Grassley, R-Iowa, and Dick Durbin, D-Ill., slammed President Trump's administration for expanding the number of H-2B visas by 15,000 this year, saying the visas, which are provided to workers in non-farm labor, would hold down wages for U.S.-born workers.

"While there may be legitimate needs among employers who rely on H-2B workers in certain sectors of the economy, a growing body of evidence shows that our increasing reliance on the H-2B visa program hurts wages for American workers and puts their jobs at risk. New research suggests that wages in some H-2B fields have been stagnant for years. The administration's decision to increase the number of H-2B visas will only exacerbate this problem," the senators said in a joint statement.

The Department of Homeland Security announced Monday that it would boost the number of visas to help U.S. businesses that would suffer "significant harm" without the workers. "As a demonstration of the administration's commitment to supporting American businesses, DHS is providing this one-time increase to the congressionally set annual cap," said DHS Secretary John Kelly.

The number of visas are normally limited to 66,000 annually, but Citizenship and Immigration Services reported in March that it had already received enough visa petitions to hit that cap.

The extent to which the workers are needed by employers has long been a hotly debated issue. Employers contended that they cannot find enough workers without the visas, but critics such as Grassley and Durbin contend that the foreign workers hold down wages, discouraging domestic ones from seeking the jobs.

"A large body of evidence suggests that our increasing reliance on the H-2B program cuts wages, pushes American workers out of jobs, and may, in some cases, discourage them from ever applying again. Indiscriminate increases in the number of H-2B workers will only exacerbate these problem," the senators told the department in a May letter.