WASHINGTON — A California congressman who heads a key congressional panel on immigration policy says a voluntary program that enables employers to check the immigration status of potential workers should be mandatory.

Rep. Elton Gallegly, R-Calif., said he is preparing to file legislation that would require employers to run the names of job applicants through the E-Verify program before they are hired.

Within seconds, the system can tell most employers whether the applicant is in the country legally and eligible to work.

“The biggest magnet for illegal immigration is jobs, so we owe it to the American people to do whatever we can to reduce the number of American jobs going to illegal immigrants,” said Gallegly, chairman of the House Judiciary Committee’s Subcommittee on Immigration Policy and Enforcement.

Democrats warned that expanding the E-Verify program or making it mandatory without also undertaking broader immigration reform could be devastating, particularly to farmers who rely heavily on an immigrant workforce.

Enforcement without broader immigration reform “simply pushes undocumented workers further into the shadows,” said Rep. John Conyers, D-Mich.

That means workers who are now paying taxes “will simply go off the books into the underground economy, which empowers bad employers and endangers everyone,” Conyers said.

E-Verify is a free, web-based program run by U.S. Citizenship and Immigration Services in conjunction with the Social Security Administration. The program is voluntary for businesses, but mandatory for federal agencies and some federal contractors.

Through the program, businesses can enter a potential worker’s name, date of birth and Social Security number in the system. The data are then checked against Social Security records and Department of Homeland Security databases to determine who is eligible to work in the United States.

Because the program is mostly voluntary, just 11 percent of the 7.7 million employers in the country use the system, said Theresa Bertucci, an associate director at Citizenship and Immigration Services.

A report issued last year by the Government Accountability Office found a number of problems with the system, including some cases in which people who are authorized to work have not been automatically confirmed by the program.

Identify fraud also remains a challenge, the report said, because employers may not be able to determine if workers are presenting genuine identification or whether employment eligibility records have been borrowed or stolen.

Rep. Zoe Lofgren, D-Calif., cited another study that indicated making the program mandatory could cost businesses $2.7 billion. Nearly all of that would be borne by small businesses, which would need to purchase computers, an Internet connection and other necessary equipment and training.

Regardless, Republicans argued E-Verify is already keeping illegal workers from taking American jobs and could become even more effective with some improvements.

Gallegly said his legislation to make the system mandatory is still being drafted. One issue to be resolved is whether a mandatory approach should be phased in, he said.