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Washington (CNN) -- Members of Congress's so-called super committee huddled in small groups behind closed doors on Capitol Hill on Friday, battling growing pessimism over their seeming inability to meet a now-imminent deadline for a bipartisan deficit reduction deal.
At least seven of the panel's 12 members, evenly split between Democrats and Republicans in the House and Senate, need to agree by Wednesday on savings of at least $1.2 trillion over the next decade. In reality, however, the timeline is even shorter. Under the law creating the committee, any proposal must be made public and evaluated for its fiscal impact by the nonpartisan Congressional Budget Office no later than Monday.
"Time is running out," panel members Rep. Chris Van Hollen, D-Maryland, and Sen. Pat Toomey, R-Pennsylvania, each said Friday morning.
"We're here. We're working. We're talking," Toomey added. "It's difficult, but it's still possible."
Both sides say they will work through the weekend to try to reach an agreement.
If the committee defies expectations and beats the clock, Congress will have one month -- until December 23 -- to vote on the deal, which cannot be amended.
A failure to pass any agreement would result in $1.2 trillion in automatic across-the-board spending cuts starting in 2013, evenly divided between defense and non-defense spending. Defense Secretary Leon Panetta warned Congress this week that such cuts could cripple the American military establishment.
While Democrats have expressed concern about deep cuts in social spending, programs such as Social Security, Medicaid, food stamps and veterans' benefits would be spared the budget ax.
Republicans -- including House Speaker John Boehner -- floated the possibility Friday of a backup plan of $640 billion in savings described as "low-hanging fruit," according to multiple sources. The plan would include roughly $540 billion in spending cuts and fees, the sources noted.
Such a measure would allow negotiators to at least claim partial success and reduce the scale of the largely unpopular automatic cuts.
Democrats balked, however, for the same reason they've also rejected larger GOP proposals: a perception that there's not enough "shared sacrifice" in the form of higher taxes on the wealthy, the sources said.
"No person could look at that (alternative plan) and say it met the test of fairness and balance," Van Hollen said.
The latest apparent failure followed a day-long blame game Thursday, with each side blasting the other for being unreasonable. Republicans continue to say Democrats aren't bending enough in terms of spending cuts; Democrats claim Republicans need to go further on tax hikes.
Democrats are "well aware of what we're willing to do," said Boehner, R-Ohio. "You can lead a horse to water, but you can't make him drink."
House Minority Leader Nancy Pelosi, D-California, claimed Democrats would prefer a "big, bold and balanced" package addressing entitlement programs and other major drivers of deficit spending, but said the "revenue piece seems to be the stumbling block for the Republicans."
Pelosi's comments appeared to be reinforced when 72 conservative House Republicans publicized their opposition to any agreement that would include tax increases. Such a large voting bloc within the GOP House majority means Boehner will need support from Democrats for any deal incorporating new revenues.
Key Republicans broke with their party's anti-tax orthodoxy this week with news of a proposal by Toomey that includes $400 billion in increased revenue, including tax hikes.
Toomey's plan would lower overall tax rates while limiting tax breaks in a way that would raise $250 billion. Republicans estimate that the reform would lead to economic growth generating another $110 billion. A change in how tax brackets are adjusted for inflation would raise another $40 billion.
The plan also includes $800 billion in spending cuts, thereby hitting the minimum threshold of $1.2 trillion in deficit reduction.
In response, Democrats initially said the additional tax revenue wasn't enough. They later partially backtracked, saying they could agree to the GOP's overall revenue number but not how the revenue would be generated.
Among other things, Democrats called for all $400 billion of the new revenue to come from increased tax collections. They also opposed a GOP demand to make permanent the Bush-era tax cuts, now set to expire at the end of 2012.
In addition, the Democratic package would include spending of about $700 billion on measures progressives believe are needed to jump-start the economy: an extension of the payroll tax cut and continued benefits for people who have been unemployed for an extended period.
It also would include money to permanently prevent cuts in payments to doctors who treat Medicare patients. Democrats want to offset those costs with money saved from winding down the wars in Iraq and Afghanistan, a move some legislators in both parties characterize as an accounting gimmick.
In return for accepting the GOP figures, Democrats also said they would not accept Republican demands to raise the Medicare eligibility age to 67 or lower cost-of-living increases for Social Security beneficiaries.
A spokesman for Boehner called the Democratic counteroffer a "backward step."
One question hanging over the talks is how financial markets would react if a deal is not reached. Conversations with stock and bond strategists indicate Wall Street has already factored in low expectations for the super committee, partially due to the near-disastrous brinksmanship that accompanied this past summer's debt ceiling debate.
If the panel can agree on $1.2 trillion in savings, the market reaction will probably be "a great big yawn," said Adrian Cronje, chief investment officer of Balentine, an investment advisory firm.
If the panel fails to reach a deal, "global and U.S. investors will continue to be disappointed in U.S. fiscal policy but will (also) look at Europe and Japan and not see governments with unassailable credit ratings in the future," said Steve Van Order, fixed income strategist at Calvert Investments
CNN's Ted Barrett, Tom Cohen, Paul Courson, Lisa Desjardins, Xuan Thai, Kate Bolduan, Alan Silverleib, Jeanne Sahadi, and Deirdre Walsh contributed to this report.