Lawmakers were expecting the House and Senate to vote Friday on the $143 billion package, which also would forestall deep cuts in Medicare reimbursements to doctors. The tax cuts, jobless coverage and higher doctors' payments would all run through 2012.
Passage seemed virtually assured in the House and likely in the Senate, though questions about its fate there were growing amid defections by senators of both parties.
Approval would hand election-year bragging rights to President Barack Obama, who made the tax cut and jobless benefit extensions a cornerstone of his September jobs package, over objections from many Republicans.
Late Thursday, Obama praised both parties for the accord and urged lawmakers to approve the measure.
"There's much more the American people need and expect from us — to help our businesses keep creating jobs, to help restore security for middle class families and to leave an economy that's built to last," the president said in a written statement that echoed themes of last month's State of the Union address.
Extending the 2 percentage-point cut in the 6.2 percent Social Security payroll tax would save around $80 monthly for someone earning $50,000 a year.
The vast number of voters who would benefit from that tax cut helped persuade many GOP lawmakers that opposition made little sense with presidential and congressional elections looming this fall. House Republicans blocked a two-month extension of the tax cut and jobless coverage in late December, only to retreat quickly under a buzz saw of opposition from conservative and GOP leaders from around the country.
With that history, most Republicans seemed ready to get the fight behind them and change the subject for the rest of this election year.
"It will allow us to move forward on our agenda in the House," said freshman Rep. Tom Reed, R-N.Y., a reference to GOP proposals for paring federal regulations and shrinking government.
"They've learned that it's politically stupid" to oppose the measure, Sen. Chuck Schumer, D-N.Y., said of GOP lawmakers.
Even so, the Senate's majority Democrats were expecting to need more than 10 Republican senators to reach the 60 votes needed to advance the measure.
Some Democratic senators were defecting because of cuts the bill would make for civil servants' benefits and health programs. Many Republicans were opposed because the measure would add $89 billion to federal deficits over the coming decade.
The reduction in the Social Security payroll tax, which is deducted from workers' paychecks, would cost $93 billion through 2022. In a sudden concession this week that made bipartisan agreement possible, House Republicans dropped their demand that the tax cut be paid for with spending reductions.
In a GOP win, coverage for the long-term unemployed would be cut from the current maximum of 99 weeks to a ceiling of 73 weeks by this fall in states with the worst job markets, with most topping out at 63 weeks.
The $30 billion cost of the extended benefits would be paid for half by government sales of parts of the nation's broadcast airwaves, half by requiring federal workers hired after this year to contribute an additional 2.3 percent of their pay for their pensions, up from the current 0.8 percent.
That increase also would apply to members of Congress, but only those who begin service as of next January — exempting every current lawmaker.
The bill also would prevent a 27 percent cut in federal payments to doctors who treat Medicare patients, a reduction that threatened to make it harder for seniors to find physicians.
That would cost about $18 billion. It would be paid for by trimming Medicare reimbursements to health care providers to cover unpaid medical bills, cutting payments to hospitals that treat large numbers of poor patients and cutting a fund created in Obama's health care overhaul for preventing diseases caused by smoking and obesity.
A House-approved measure letting states test unemployment benefit applicants for drug testing was pared back, permitting the tests only for people who lost their jobs due to drug use or whose new jobs would require such tests.
Those seeking unemployment coverage would have to show they are actively seeking work, but another GOP-backed provision forcing them to pursue high school equivalency diplomas was abandoned.
Associated Press writer Andrew Taylor contributed to this report.