"With the Illinois Act, the state ... is attempting to indirectly regulate the U.S. government by imposing state standards on a federal program that must be satisfied before Illinois employers are permitted to enroll," the 12-page complaint states. "Congress also made it clear its objective and purpose to encourage broad participation in the Basic Pilot Program by instructing DHS to expand the program to all 50 states."

The suit asks for the state law to be declared invalid and seeks an injunction on the ban. Employers using the program submit job applicants' identification information into Social Security Administration and Homeland Security databases on the Internet to confirm their work eligibility. In most cases, the check takes a few seconds, according to Homeland Security. If the program cannot confirm the work eligibility of an applicant, it issues a "tentative nonconfirmation" notice that can take days or weeks to resolve.

Supporters say the program helps employers keep illegal immigrants off employee rolls, but critics note it has an accuracy rate of between 80 percent and 95 percent, which they say marks it as too unreliable and results in some applicants being wrongly dismissed. The Illinois ban was to remain in effect until the program results were 99 percent accurate. The ban also exempts employers who undergo training in the program and receive "anti-discrimination notices" from the U.S. Department of Justice and Illinois Department of Human Rights. About 750 Illinois businesses used E-Verify before the ban took effect, according to the complaint.

The Illinois Coalition of Immigrant and Refugee Rights, a Chicago-based advocacy group, said they thought Homeland Security was wrong in considering the voluntary E-Verify program a federal mandate.

"The verification program is voluntary for nearly all private employers; they are not required to participate in these verification programs to begin with," the statement reads. "The law does not interfere with any federal obligation because for most employers no such obligation exists."

But the department's actions may already have had an effect, as some state lawmakers started backing away from the amendment after learning of the lawsuit.

"We should take a second look at that bill," said Rep. Ruth Munson, R-Elgin, who was one of 76 state House members who voted to pass the amendment in April. "If DHS thinks it's an important tool, the state ought to step back and look at what it does. Maybe the law needs to be repealed or changed."