– Only a handful of states require all employers to screen workers to make sure that they are in the country legally, but that could change soon.

Spurred by the Trump administration’s crackdown on illegal immigration, Congress is set to debate a national E-Verify mandate that would require every U.S. employer to use the federal online service to screen all new hires and for the first time allow the screening of current employees.

Seven states in the South (Alabama, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee) and two in the West (Arizona and Utah) already require private employers to use E-Verify. But enforcement has been inconsistent, largely because state officials can’t access E-Verify records to make sure employers are checking new hires. Advocates say a federal mandate could boost compliance.

“It’s cumbersome for states to know whether employers are actually complying,” said Madeline Zavodny, an economics professor at Georgia’s Agnes Scott College.

Between October 2016 and June 2017, 70 percent of all hires in the South were screened with E-Verify, compared to only 39 percent in the Northeast, where no states require E-Verify, according to an analysis of screening data.

Nationally, 57 percent of jobs are now screened with E-Verify, up from half in 2015 and about 30 percent in 2010.

But a study of seven of the nine E-Verify states released earlier this month by the Federal Reserve Bank of Dallas suggests that state mandates have had mixed results. The study did not include Louisiana or Tennessee, because those states have allowed businesses to use screening methods other than E-Verify.

Dallas Fed researchers determined that the mandate reduced or slowed the growth of the population of unauthorized immigrants in Alabama, Arizona, Mississippi and Utah.

But there was no significant change in Georgia, North Carolina and South Carolina, even though Georgia and South Carolina have done more than most other states to enforce the law. (Georgia did have fewer unauthorized workers than projected without the law, but the total number of unauthorized immigrants in the state was unchanged.)

In 1986, the federal Immigration Reform and Control Act made it illegal to knowingly hire unauthorized workers. But in states that don’t mandate E-Verify screening, employers may hire workers with falsified paperwork and still comply with federal law, because they are not knowingly violating it. E-Verify cannot be used to screen existing employees — only new hires.

Immigration advocates say they oppose a national E-Verify mandate unless it includes help for the unauthorized workers who are already here.

“There are millions and millions of people in that situation and millions of employers who depend on their labor,” said Jessie Hahn, an employment policy attorney at the National Immigration Law Center.

Without legal work status for those already here, Hahn said, E-Verify means “shifting people into the underground economy where they’re not paying taxes and they’re more vulnerable to abuse.”

Some longtime opponents of E-Verify recently have changed sides. Some business groups now say they would prefer a national standard to rules that vary by state.

Many small businesses such as restaurants, hotels and janitorial services have changed their minds because they’re afraid of being raided and closed down under an immigration crackdown, said Tamar Jacoby, president of ImmigrationWorks USA, a federation of small business owners that advocates for an expansion of legal immigration and guest worker programs.

“It will change the way companies do business, but I think Americans in general want to move toward a legal workforce,” Jacoby said. “The employers who use it have been telling other employers that, yes, it’s a hassle at first and takes some time to set up, but then you can sleep at night.”

Despite the Trump administration’s support for a mandate, opinion is mixed, even among conservatives. “It’s an intrusive labor market regulation that raises the cost of hiring,” said Alex Nowrasteh, an immigration policy analyst at the libertarian Cato Institute.