Under Mr. Obama’s executive order creating the panel of 12 members of Congress and six private citizens, 14 of the 18 commissioners must agree in order to send any package to Congress for a vote in December. The Senate majority leader, Harry Reid of Nevada, and Ms. Pelosi, who will remain the speaker until January, have promised in writing that the Senate would vote first and, if it approves a plan, the House would vote.

“I think it’s possible” that 14 members will agree, said Senator Tom Coburn, a conservative Oklahoma Republican who worked closely with the chairmen on proposed reductions from the military and in so-called tax expenditures, the myriad tax breaks for individuals and businesses that cost more than $1 trillion a year. “You don’t know until you see what the final plan is.”

In five hours of deliberations on Wednesday, the commission did not discuss the plan’s particulars much but instead talked at length about whether a lame-duck Congress would have time to write specific legislation and then vote, members said in interviews. It was unclear, they said, whether that was a sign other members thought the commission actually could reach agreement, or whether they were hiding behind concerns about legislative procedures to avoid tough policy decisions.

“At least people stayed in the room,” Andy Stern, the former president of the Service Employees International Union, said in an interview, recalling his concerns and others’ that Republicans would walk out if taxes were on the table and Democrats if Social Security and other spending programs were.

Right now the biggest issue facing the lame-duck Congress is whether to extend the Bush-era income tax cuts, which expire Dec. 31, for all taxpayers, as Republicans want, or for income below $250,000, as Mr. Obama and Democrats want. The Bowles-Simpson plan includes one option that assumes only the lower-income rates are extended and another that ends all Bush tax rates and replaces the tax code with simpler, lower rates and many fewer tax breaks.

Extending all the Bush tax cuts through 2020 would add more than $4 trillion to the debt  coincidentally, about the same amount that the chairmen’s painful options are designed to cut in the same time frame.

Their proposed simplification of the tax code would repeal or modify a number of popular tax breaks  including the deductibility of mortgage interest payments  so that income tax rates could be reduced across the board. Under one option, individual income tax rates would decline to as low as 8 percent for the lowest income bracket (it is now 10 percent) and to 23 percent for the highest bracket (now 35 percent). The corporate tax rate, now 35 percent, would be reduced to as low as 26 percent.