Sen. Jeff Sessions (R-AL) is pushing back against an effort that would permanently and dramatically expand the annual cap on low-skilled, temporary foreign workers allowed in the U.S. each year.

In a letter to Senate appropriators first reported by Politico, Sessions argues against a bipartisan group of lawmakers’ recent call to make permanent the elevated level of low-skilled guest worker visas contained in last year’s omnibus spending bill.

That provision allowed for the exemption of so-call “returning workers” — or foreigners who had guest worker H-2B status for the previous three fiscal years — from the annual H-2B cap of 66,000.

“It is my understanding that certain members of the Senate seek to make this ‘returning worker’ exemption permanent. Doing so would be a grave error,” Sessions wrote in the letter, obtained by Politico.

“Fundamentally, a cap must be a cap,” he continued. “Not counting H-2B workers against congressionally defined caps is as misleading to the American people and it is detrimental to wages and job opportunities of American workers.”

The H-2B provision contained in the omnibus bill essentially allowed for a four-fold increase in the number of seasonal foreign workers admitted to work in the U.S. in low-skilled industries.

Sessions argued the high cap would have serious impact on American workers.

“[M]illions of Americans who are currently unemployed or underemployed directly compete for the jobs that are often filled by H-2B workers: i.e. landscapers, hotel workers, loggers, construction workers, amusement and recreation attendants, waiters or waitresses,” Sessions wrote.

“Congress may not be able to flip a switch and correct all of the damage that has been inflicted on these hardworking men and women, but we can ensure that we do not inflict further harm upon them by adding to the labor force with cheap foreign labor,” he added.

According to the Alabama lawmaker, the Congressional Budget Office’s analysis of the provision’s effect — which found that it would increase the number of additional workers in FY 2016 by 8,000 — was “flawed” and “ignored historical trends and available data.”

“Undoubtedly, including this provision for an entire fiscal year will result in substantially more temporary workers in FY 2017,” Sessions concluded. “This is an important issue and it says something about the priorities Congress has. It is desirable that we avoid an even more contentious debate over this issue than last year.”