WASHINGTON — The Senate voted Tuesday to fine employers who hire illegal immigrants up to $20,000 for each unauthorized worker, providing teeth to a broad immigration bill before sending it to a final vote later this week.

Employers would have to check Social Security numbers and the immigration status of all new hires within 18 months after money is provided to the Homeland Security Department to expand the electronic system for screening workers.

"This is probably the single most important thing we can do in terms of reducing the inflow of undocumented workers, making sure we can enforce in a systematic way rules governing who gets hired," said Sen. Barack Obama, D-Ill.

The amendment passed 58-40. Opponents said the verification system would take years to implement and complained that workers deemed illegal could still hold onto jobs until their appeals are exhausted.

Employers who don't use the new computerized system could be fined $200 to $600. The system would include information from the Social Security Administration, the Internal Revenue Service and Homeland Security Department.

The $20,000 fines for hiring illegal immigrants once the new screening system is in place would be double the present level. Repeat violators could be sentenced to prison terms of up to three years.

The House passed a bill in December that would impose fines on employees of undocumented workers ranging from $5,000 to $40,000. But, unlike the Senate bill, the House measure would require employers to screen all employees — an estimated 140 million people — instead of only new hires.

Majority Leader Bill Frist, R-Tenn., scheduled a test vote for today that sets up the bill's final passage, likely Thursday. Its most controversial provision would put more than half of the nation's 12 million illegal immigrants on a path toward citizenship without ever having to leave the United States.

Critics call that amnesty, and Republican leaders refused to even allow it to be considered in the bill the House passed in December.

Rep. Mike Pence, R-Ind., who heads a group of 100 conservatives in the House, said Tuesday he plans to offer a bill this week that would let employers rehire illegal workers now on their payrolls after they have returned home and applied for a new "W" visa to return.

"The solution is to set up a system that will encourage illegal workers to self-deport and come back legally as guest workers," said Pence, who earlier voted for the enforcement-only House bill.

The Senate defeated an effort Tuesday by Sen. Dianne Feinstein, D-Calif., that would have let all illegal immigrants remain, in contrast to the Senate compromise that would require more than one-third of them to leave.