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The current bill again promises to crack down on employers with fines and the potential for jail time.

Numbers are difficult to come by, but the Center for Immigration Studies estimates that 55 percent to 60 percent of illegal immigrants are working on the books, which means their taxes are being paid — though they are credited to bogus or stolen Social Security numbers.

The Obama administration has stepped up enforcement against employers compared with its predecessor, conducting far more reviews of businesses’ immigration records.

In 2006, for example, George W. Bush’s administration didn’t issue any final orders imposing penalties on businesses. But the Obama administration in 2011 issued 385 final orders, for a total of $10.5 million in fines.Mr. Krikorian said one question is whether it would be worth the cost to the government to try to go after business scofflaws for back taxes or other penalties. He said that would be difficult given the short time frames written into the Senate bill. If Congress writes a longer-term bill that requires Internal Revenue Service audits of illegal immigrants and businesses, he said, it might be possible.

The Senate Judiciary Committee will begin voting on amendments to the bill next week, once lawmakers return from a weeklong vacation.

The business amnesty could prove tricky to navigate when the bill comes to the Senate floor. Amendments imposing penalties on businesses that broke the law could be attractive for many lawmakers but could upset the delicate balance of the bill.