(CNN) - A compromise Senate plan to pay for the cost of extending unemployment benefits ran into resistance Thursday from a group of Republicans whom the agreement was supposed to attract.
Those GOP senators, who had provided critical votes to Democrats to begin debate on the bill, said they wanted to make changes to the offsets and offer other amendments.
But Senate Majority Leader Harry Reid said he would not allow Republican amendments to the bill, angering GOP senators.
"I don't think it's going to fly," said Sen. Dan Coats, R-Indiana, a soft-spoken Senate veteran who had a testy floor exchange with Reid over the amendment issue. "Right now I will vote against this bill."
The 55-member Senate Democratic caucus can't pass the bill without help from Republicans to clear procedural hurdles that require 60 votes to pass.
"I don't think it's sustainable to shut everyone else out because, frankly, he needs six of us to say yes," said Sen. Rob Portman, R-Ohio.
Earlier Thursday, there seemed to be momentum for the compromise that would have offset the $18 billion cost of the benefits by keeping cuts to Medicare providers and by preventing people who get Social Security disability benefits from also getting unemployment checks.
The deal was brokered by Senate Democrats and GOP Sen. Dean Heller of Nevada, one of the six Republicans who voted to take up the bill. But support for it quickly faded on policy and political grounds, especially after it became clear the other five Republicans were not part of the negotiations.
A key procedural vote on the compromise is scheduled for Monday.
Senators who were involved in the busy day of negotiations and recriminations said that they hoped cooler heads would prevail before Monday and that a new deal could be reached.
"It's not DOA yet," said Portman who said he would work over the weekend to try to reach an agreement.
"I'm just trying to keep everyone working, calm, and trying to get a solution," said Sen. Susan Collins, R-Maine.
"While today's session was a lot of bad blood, there is still a view on both sides that maybe we can get something done," said Sen. Chuck Schumer, D-New York. "Maybe, underlined."
CNN"s Lisa Desjardins contributed to this report.