As Congress reconvenes this week, businesses and their lobbyists are gearing up to fight a series of proposed changes to immigration law that they say will complicate hiring.

Their target is Senate legislation that would legalize an estimated 12 million undocumented immigrants but also significantly change employment law. Among employers’ top concerns is a provision imposing criminal liability for employers if their subcontractors hire illegal workers. The legislation would also increase civil penalties for employers caught hiring illegal workers.

The maximum criminal penalty for a pattern of hiring illegal workers would increase to $75,000 for every undocumented worker from $3,000.

Under the bill, a business might have to vet the employment files of its subcontractors, which lawyers said would be an onerous task.

“They are not your people, [and] it is much more difficult to ensure there are controls in place at another company,” said Daniel Brown, a lawyer at Paul Hastings in Washington and a former general counsel for the Department of Homeland Security. “That’s a huge burden on a huge number of employers.”

Businesses say the legislation, which senators are to resume debating this week, could result in a hiring nightmare because of a provision calling for an expansion of a sometimes-faulty worker database.

“We’ve got grave concerns,” said Susan Meisinger, chief executive of the Society for Human Resource Management, which has 200,000 members.

The bill would require that the nation’s 7.5 million businesses begin within 18 months to use a screening system run by the Department of Homeland Security called Basic Pilot to vet new hires. Within three years, businesses would have to verify every worker.

The businesses, working under an umbrella organization called the HR Initiative for a Legal Workforce, have run advertisements in Capitol Hill publications decrying the federal database. They have also been making the rounds of congressional offices, hoping to sway legislators and find a champion for their cause.

The database question is a “huge sleeper issue,” the director of education and workforce policy for the National Association of Manufacturers, Heath Weems, said. Employers like to point out that meatpacker Swift & Co. was voluntarily using the Basic Pilot screening program when a Department of Homeland Security raid last year discovered that hundreds of workers were illegal. Employers have complained that Basic Pilot’s data is often out of date, particularly when people have changed their names or immigration status. If the database doesn’t recognize the new name, the employee cannot be legally hired.

The White House and the senators who are shepherding the immigration legislation have said tougher employment verification requirements are essential to stem illegal immigration. The Bush administration released a statement last week saying that Basic Pilot will be up to speed for the task and that it verifies 92% of workers as eligible for employment within three seconds. It flags potential problems with the others at the same speed.

Business groups are also battling provisions that restrict their ability to hire foreign workers. Employers of skilled workers worry that changes to the visa distribution program proposed by the bill would limit their ability to recruit internationally. They also say the legislation does not substantially increase visas available to skilled workers under existing law.

Robert Hoffman, the top lobbyist at Oracle, plans to send a letter to every senator today asking for support of an amendment by Senator Cantwell, a Democrat of Washington, and Senator Cornyn, a Republican of Texas. The amendment would remove restrictions that force employers to recruit foreign workers only from a government-approved pool and exempt from visa limits foreigners who have earned advanced degrees in America.

Employers of unskilled workers approve of the Senate’s proposal to give the nation’s 12 million illegal workers employment papers but say a guest-worker program that offers business 200,000 visas to fill specific holes in the U.S. economy is too small.

“If there is no economic rationale for the number, then people will continue to come illegally,” the director of federal affairs at the National Roofing Contractors Association, Craig Silvertooth, said.