The majority leader, Senator Harry Reid of Nevada, said he believed that Democrats could muscle the stimulus bill through with at least two Republican votes. But late Thursday he said he would give the bipartisan group until Friday to reach a deal. If no deal is reached, he said he would call for procedural vote on Sunday aimed at moving to final vote.

The efforts of the bipartisan group, which at one point numbered about 20 senators, essentially tied Mr. Reid’s hands, giving him little choice but to allow time for a compromise measure to emerge. The behind-the-scenes brokering distracted from the Senate floor where formal debate continued and senators at times traded angry barbs.

The Democrats will need the support of at least two Republicans and probably more to win passage of the stimulus bill, which for procedural reasons will require 60 votes. The Democrats now hold 58 seats, but only 57 have been voting this week. Senator Edward M. Kennedy of Massachusetts has been absent because of illness.

Image Harry Reid, the Senate majority leader, center, speaking on Capitol Hill on Thursday with, from left, Senators Charles E. Schumer and Richard J. Durbin. Credit... Ruth Fremson/The New York Times

Ms. Collins is clearly one Republican they are counting on. But on Thursday evening she said she would not vote for the measure in its current form and wanted further changes in spending programs. “My goal is to be able to support a bill,” Ms. Collins said.

At issue is the huge package of tax breaks and government spending intended to halt the economy’s downward spiral and spur a recovery that is the centerpiece of Mr. Obama’s agenda. Congressional Democratic leaders have promised to adopt the measure and send it to the White House for his signature by the end of next week.

Lawmakers said they can still meet that goal. But once the Senate acts, the bill must still be reconciled with a version passed by the House last week, and some House Democrats said Thursday that they were increasingly alarmed by reports of efforts in the Senate to remove programs that they believed would lift the economy.