(CBS) -- With the collapse of talks between the White House and House Republicans, all eyes are now on the Senate to broker a deal that reopens the government and lifts the debt ceiling before the government reaches the end of its borrowing authority - and potentially defaults on the nation's debt - on Thursday.
The Senate convened for a rare weekend session on Sunday, 13 days into the government shutdown and only one day after two separate plans to resolve the standoff ran aground in the upper chamber. One, a proposal offered by Democrats that would raise the debt ceiling through 2014 without any legislative add-ons, was blocked by a unified caucus of 45 Senate Republicans.
The other was a plan spearheaded by Sen. Susan Collins, R-Maine, in consultation with moderate Democrats, that would have raised the debt ceiling through January and reopened the government for six months in exchange for a two-year delay of the tax on medical devices levied by Obamacare. That plan was brushed aside by Senate Majority Leader Harry Reid, D-Nev., who said it conceded too much in pursuit a short-term solution.
The burden of finding a workable deal has now fallen squarely on the shoulders of Reid and Senate Minority Leader Mitch McConnell, R-Ky., who met on Saturday morning for talks that Reid characterized as preliminary but encouraging.
For the most part, senators of both parties on Sunday voiced a measured sense of optimism that Congress would be able to steer its way out of the mess before the debt ceiling is breached, but the parameters of any potential deal remain very much uncertain.
Republicans, Obama again at impasse over government shutdown: Now what?
Sen. Schumer optimistic that Senate negotiators can end the government shutdown
Republicans expressed frustration that Collins' proposal had been shelved by Democratic leadership, accusing the Democrats of recklessly pressing their advantage as the deadline approaches.
"I did think we had a framework for a plan," said Sen. Kelly Ayotte, R-N.H., on "Face the Nation." "We had six Democrats, six Republicans. Something that would get the government open, address the debt ceiling also. Some things we could agree around Obamacare. ... We were close and then the Senate Democratic leadership, and I believe the White House, pulled back."
Sen. John McCain, R-Ariz., said on "Face the Nation" that Democrats had been emboldened by poll numbers showing the GOP bearing the brunt of blame for the crisis, and had therefore decided to stiffen their negotiating posture at the last minute.
"I'm disappointed that twice they were close to a deal and the Democrats moved the goalpost in light of the polling data," he said.
Sen. Bob Corker, R-Tenn., said that while Republicans entered the shutdown debate with unreasonable demands to scuttle key portions of Obamacare, it is now Democrats who are guilty of bargaining too aggressively.
"Republicans started off in a place that was an overreach, to defund a law that was central to the president's agenda was not achievable," he said on "Fox News Sunday." "Now Democrats are I think getting too cute. They now are overreaching."
In a statement released Sunday, McConnell called on Democratic leaders to reconsider their opposition to Collins' proposal. "There is a bipartisan plan in place that has the support of Democrat and Republican Senators. It would reopen the government, prevent a default, provide the opportunity for additional budget negotiations around Washington's long-term debt, and maintain the commitment that Congress made to reduce Washington spending," he said. "It's time for Democrat leaders to take 'yes' for an answer."
Still, some lawmakers expressed hope that the framework of a deal constructed by Collins could be resuscitated and reworked to eventually reach a solution.
"I think the plan and work that we're doing is positive," said Sen. Amy Klobuchar, D-Minn., with whom Collins was consulting on her plan, on CNN. "I think what Sen. Reid meant is that he wasn't going to accept every little detail of it. But I think he knows that there are some positive things in that plan that are very good. For instance, we are talking about opening the government again and doing it in a smart timeframe."
Collins herself said on CNN that lawmakers continued to express interest in exploring her offer, even after Reid had sidelined it, signaling that it might yet provide the basis for a deal.
"Just last night, I had two more Democrats and a Republican contact me to offer suggestions and say they wanted to be part of my group," she said. "I do believe we're going to see a resolution this week."
While Reid and McConnell have offered scant details about the scope of their negotiations, there is some speculation that, in addition to raising the debt ceiling and reopening the government, a final deal could include an agreement to reexamine the spending cuts enacted under sequestration.
"Look, neither Democrats nor Republicans like the sequester," said Sen. Chuck Schumer, D-N.Y. on "Face the Nation." "The dispute has been how to undo sequester. Republicans want to do it with entitlement cuts. In other words, take entitlement cuts and then put that money into undoing at least part of sequester. Democrats want to do it with a mix of mandatory cuts, some entitlements and revenues. And so how do you overcome that dilemma? We're not going to overcome it in the next day or two. But if we were to open up the government for a period of time that concluded before the sequester took place, which is Jan. 15, we could have a whole bunch of discussions."
"I am more optimistic than most we could come to an agreement," he added. "That was one place where the House Republicans and the president were not at total loggerheads."
Sen. Rand Paul, R-Ky., warned on CNN, however, that it would be a mistake for Democrats to undermine the deficit reduction accomplished by sequestration.
"I think the one thing I cannot accept, and the one thing that I think is really not even a compromise at all, is the Democrats want to exceed the sequester caps, these things that we put into law to restrain spending already," he said. "I can't imagine you're going to get Senate Republicans to vote for something that exceeds the sequester caps. I think it's a huge mistake for the country. The number one problem we face is our debt. We have to do something about our debt."
On the Senate floor Sunday, Reid disavowed any talk of exceeding the spending limits imposed by sequestration. "Any talk of breaking the caps is not coming" from Democratic leaders, he said
While the locus of the action is now in the Senate, any eventual deal must also be approved by the House before it goes to President Obama's desk. Some expressed concern on Sunday about whether the House would be able to muster the support to pass whatever the Senate eventually sends across the capitol.
"I'm worried about a deal coming out of the Senate, that a majority of Republicans can't vote for in the House, that really does compromise [House] Speaker [John] Boehner's leadership," said Sen. Lindsey Graham, R-S.C., on ABC's "This Week." "And after all this mess is over, do we really want to compromise John Boehner as leader of the House? I don't think so."