Republican and Democratic senators on Wednesday urged the Trump administration not to approve an additional number of foreign guest workers this year without significant reforms to the controversial program -- which they say incentivizes employers to hire foreign workers over their American counterparts.

“While we understand the needs of employers who legitimately rely on seasonal H-2B workers if American workers cannot meet the demand, we continue to have concerns about the harmful impact that the program has on both American workers and foreign guest workers,” says the letter signed by Sens. Chuck Grassley, R-Iowa., Tom Cotton. R-Ark., Dianne Feinstein, D-Calif., Richard Blumenthal, D-Conn., and Richard Durbin, D-Ill.

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The H-2B visa program gives temporary legal status to non-agricultural seasonal workers in areas such as landscaping and service-industry jobs in restaurants and hotels. The vast majority of the visas go to Mexican workers. The number of such visas in a single year is capped by Congress at 66,000, but the Trump administration has raised that cap significantly each year. Last year it was increased by an addition 30,000 workers.

In the letter to Acting Department of Homeland Security Secretary Chad Wolf and Labor Secretary Eugene Scalia, the lawmakers say that studies have shown that the program hurts both American and immigrant workers alike.

“Studies have shown that the H-2B visa program leaves immigrant workers vulnerable to wage theft, abuse, and trafficking. Because they are often at the mercy of their employers, H-2B workers may also be too scared to speak out against poor working conditions. They may also have difficulties accessing the justice system to protect themselves from employer retaliation if they do speak out,” the letter says.

“These realities of the H-2B program, as it operates today, incentivize unscrupulous employers to hire H-2B workers instead of American workers and create poor working conditions for immigrant workers and American workers alike,” it says. “Therefore, absent significant regulatory and legislative reforms to the program, we do not believe that an increase in the number of H-2B visas is in the interests of either American workers or H-2B visa holders."

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Proponents of increasing the visa caps argue that a booming economy with a low employment rate requires loosening restrictions so companies can get the workers they need, particularly during labor-intensive seasonal periods. Trump, who ran his 2016 campaign in part on putting American workers first, has not commented on the H-2B program specifically. However, he recently expressed support for companies seeking to bring in more foreign labor in the context of the separate H-1B visa program in which high-skilled workers are brought in from abroad, mainly for the tech industry.

“We don’t have enough of them and we have to be competitive with the rest of the world too,” Trump told Fox News’ Laura Ingraham last week. “The companies want to hire these people. ... They want to hire smart people and those people are thrown out of the country -- we can’t do that.”

But it hasn’t provided assurance for immigration hawks, who object to increases in both H-1B and H-2B visa programs and say such increases clash with Trump’s ‘America First” stance when it comes to protecting American workers.

The Federation for American Immigration Reform (FAIR) wrote to Acting Homeland Security Secretary Chad Wolf last week, arguing that increasing H-2B caps should be a “last resort labor option when it’s proven that no local workers are available.”

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“The tight labor market is creating incentives for employers to increase wages, offer new benefits and hire people they previously would overlook,” FAIR President Dan Stein said, citing research that suggests an influx of H-2B visa holders are paid less than Americans performing the same jobs -- which results in depressed wages overall.

“Flooding the job market with cheap, low-skilled foreign workers is a betrayal of the very workers President Trump has consistently pledged to protect,” he said.

In the letter sent Wednesday, the lawmakers request the administration keeps “careful records” of assessments made in raising the visa cap, as they will be requesting “a detailed written report and staff-level briefing to the Senate Judiciary Committee justifying and explaining how you arrived at that determination.”