U.S. Immigration and Customs Enforcement agents serve an employment audit notice at a 7-Eleven in Los Angeles in January 2018. (Chris Carlson/AP)
Columnist

President Trump is making some conservatives nervous. During an interview that aired Sunday, he expressed concern about E-Verify, the system that checks whether someone can work in the United States legally, noting that it could become onerous for employers.

Trump should recognize quickly that, among his supporters, mandatory use of E-Verify would be a nonnegotiable element of any immigration deal.

Conservatives have long opposed any grant of legal status — or “amnesty” — to undocumented immigrants, but most would grudgingly support a deal including amnesty if they could be certain that it would solve the illegal immigration problem once and for all. For them, mandatory use of E-Verify must be part of it.

Mandatory E-Verify is crucial because it closes the loophole that allows the illegal immigration issue to resurface in the first place. The 1986 Simpson-Mazzoli Act provided amnesty for millions of undocumented immigrants already in the United States. In exchange, it required employers to request documentation of a job applicant’s legal status and made it illegal to knowingly hire people who could not legally work in the United States. In theory, the problem was solved.

Crucially, however, the law did not require employers to verify that an applicant’s documents were legitimate. Employers merely had to accept those documents and file an I-9 form for employment eligibility verification with the federal government. Without a mandatory verification process, the employer could say they did not know an applicant was in the country illegally, allowing them to employ undocumented workers and escape liability. The trade in phony Social Security cards and other forms of identification required on the I-9 form has since exploded.

Trump’s concerns about E-Verify contradict what he said on the campaign trail in 2016. They also draw attention to the past hiring practices of the Trump Organization, which only started to use E-Verify at all of its locations this year after news reports noted that it had employed undocumented immigrants. The discrepancy between what the president said and what his companies had done was too embarrassing to let it continue. But that discrepancy throws light on the loophole that has allowed illegal immigration to flourish.

Building the wall and enhancing border security are not enough. Barriers can reduce the flow of illegal immigrants, but the flow will never stop unless we address the demand for illegal immigration. That’s because the difference between what an immigrant could earn in his home country and in the United States is massive, and because U.S. employers can save thousands of dollars per employee by hiring illegal immigrants.

Take El Salvador. The minimum wage there differs by occupation, but it is about $2,900 a year for retail or industrial employees and $1,300 for agricultural laborers. The U.S. federal minimum wage is $7.25 an hour, or about $15,000 a year for 40-hour weeks. Some states and cities set higher minimums, such as $14 an hour in the District. A Salvadoran making D.C.’s minimum wage can earn nearly $28,000 a year, almost 10 to 25 times what he or she earns back home. No wonder there are nearly 200,000 Salvadorans living in the D.C. metro area.

That earning differential supports the economies of several Central American nations. Even after accounting for the higher cost of living, Salvadorans living in the United States earned enough in 2016 to send more than $4.6 billion back home. That was roughly 17 percent of the country’s entire gross domestic product. Last year, that figure was closer to 21 percent. The money Hondurans living abroad sent home constituted nearly 20 percent of their nation’s GDP, and Guatemalans sent home more than 12 percent of their GDP. No wall is going to stop these people from trying to come here and support their families back home.

U.S. employers will always want to hire these people if they can. Their labor supply keeps them from having to raise wages to a level that could attract Americans to do the same work. The only way to ensure that illegal immigration stops and wages for low-skilled Americans rise is to prevent employers from hiring illegal labor. Mandatory E-Verify is the best and most reliable way to do that.

Trump’s business background makes him vulnerable to employers’ pleas. Perhaps mandatory E-Verify would be cumbersome, as he claims, and it might mean that many positions stay open longer until an employer can find a qualified and legal applicant. But that’s a feature, not a bug. Just as many U.S. companies rely on Chinese imports and howl at Trump’s tariffs, many U.S. employers rely on illegal labor and howl at mandatory E-Verify. Trump needs to be as firm on E-Verify as he is on tariffs and show U.S. business that they need to change.

Candidate Trump stood firmly on the side of U.S. workers who had been economically devastated by competition from Chinese trade on the one hand and illegal immigration on the other. President Trump needs to keep his promises to those workers — and that means insisting on mandatory E-Verify.

Read more:

Henry Olsen: Trump’s immigration plan has one key fault

Daniel Okrent: Kushner’s immigration plan is a version of a discriminatory effort from more than a century ago

Juliette Kayyem: An Instagram-worthy immigration plan

The Post’s View: How both Trump and Democrats can win on immigration

Eugene Robinson: Trump invented an immigration crisis to further his most consistent goal