The law provides an exemption from the 6.2 percent Social Security payroll tax for every worker who has been unemployed at least 60 days and is hired after Feb. 3 through the end of this year. Employers will get an additional $1,000 income tax credit for new employees retained for at least a year. The Senate estimated the measure will cost $13 billion over 10 years, though most of the cost will be in the first two years.

While employers are expected to claim the credit for as many as five million workers, Timothy J. Bartik, senior economist at the W. E. Upjohn Institute for Employment Research, estimated that the new law would create no more than 200,000 jobs that would not otherwise have existed.

“The design of the Senate bill passed today is particularly beneficial to high-turnover companies, like restaurants, which in a typical year might replace fully half of their work forces,” said John H. Bishop, an economist in the School of Industrial and Labor Relations at Cornell. “To get the hiring subsidy, it doesn’t require that a company actually grow.”

While the measure won support from some Republicans, others in the party were skeptical that it would help create jobs, or said they were distressed at its cost. “This isn’t so much a jobs bill as it is a debt bill,” said Senator Judd Gregg, a New Hampshire Republican.

The measure is partly paid for by tightening rules on offshore tax havens, but some costs would be added to the deficit.

The bill was the first in some time to draw a sizable bloc of Republicans in the polarized Senate, an indication that some Republicans acknowledge they cannot stand in blanket opposition to Democrats on economic matters. Some Republican support was due to the push for more road and bridge building, which creates good-paying jobs for idle construction workers.

“The funding in this bill sends the message to struggling states like Ohio that they can move forward with shovel-ready transportation projects — projects that will put people back to work quickly and contribute to economic growth,” said Senator George V. Voinovich, Republican of Ohio.