Negotiations over the extension of unemployment benefits for the long-term jobless suffered a major setback in the Senate on Thursday, as competing plans to pay the costs of the extension began to emerge.

Democrats offered a roughly $18 billion proposal that would extend the benefits through most of 2014, and Senate Majority Leader Harry M. Reid (D-Nev.) scheduled a showdown vote on the new proposal for next Monday.

Democrats need at least five GOP senators to cross the aisle and support the plan, but they appear to lack the support of several of the key six Republicans who previously supported advancing the bill and have now said they are very likely to vote against the new proposal.

“We’ve done everything the Republicans wanted,” Reid said Thursday. “Now is the time to fish or cut bait.”

The new Democratic plan would extend the emergency unemployment program until mid-November (the original proposal was for three months). By law, most unemployment benefits expire after 26 weeks, but the current federal plan allows for an additional 47 weeks of insurance in the states hardest hit by unemployment.

Who are the long-term unemployed?

Reid’s proposal would reduce the additional jobless benefits to 31 weeks in the hardest-hit states, giving an unemployed worker in a state like Nevada or Rhode Island — with the highest jobless rates of 9 percent — up to 57 weeks of state and federal insurance. States with better economies would get shorter federal extensions for their federal unemployment benefits.

The issue has taken center stage early in the new year because the federal law that provides benefits to the long-time unemployed expired on Dec. 28. Benefits were shut off immediately for 1.3 million jobless workers, and more are slated to lose benefits throughout the year as they run up against their limits.

GOP support for Reid’s plan is tepid. Of the six senators who voted Monday with 54 members of the Democratic caucus to advance the legislation, Sens. Daniel Coats (R-Ind.) said Thursday that he was certain to vote against Reid’s proposal and Sen. Rob Portman (R-Ohio) said he was likely a “no” vote.

After Reid spoke, Portman criticized the plan for not including his proposal to save money by barring the jobless from claiming benefits from the federal disability insurance program and the unemployment program.

Reid’s plan would bring savings, but Portman said his version would bring more than $5 billion in savings. He said that Reid had not involved him in the negotiations and that he would try to work over the weekend to reach a different deal with Democrats.

“We haven’t had the chance to sit down and talk about this,” Portman said, accusing Democrats of dropping in their amendment and cutting off bipartisan negotiations.

Some remained optimistic that discussions could continue over the weekend and that a framework for a deal could come together before the Monday evening vote. “I don’t think it’s irreparable at this point,” Sen. Charles E. Schumer (D-N.Y.) told reporters.

Without such a deal, Democrats will lack enough support to clear a 60-vote hurdle for a filibuster.

Coats expressed dismay at Reid’s move to shut down the amendment process and leave the GOP in a take-it-or-leave-it position next week.

Regardless of the outcome Monday, House Republicans signaled skepticism because most of the budget savings come from what they consider a gimmick. Reid’s plan would draw $17 billion in savings by extending for one additional year portions of the mandatory spending cuts, known as sequestration.

That would represent a cut to funds for Medicare providers, but it would not be implemented until 2024 — a frequent complaint from House conservatives, who dislike it when spending in the near term is offset by cuts that will happen years from now.

“That sounds like something the Senate would try to do. But I’m not going to opine,” House Majority Leader Eric Cantor (R-Va.) said.

Cantor then urged lawmakers to focus afresh on policies aimed at “getting people back to work” and to “reject the new normal of all this chronic unemployment.”