Declaring that the legislation was on "life support," Sen. Edward M. Kennedy (D-Mass.) said, "We're at a very dangerous time."
Republican lawmakers were less pessimistic but agreed that negotiations over a plan to provide prescription drug coverage to 40 million seniors and disabled people had reached a critical stage.
"We decided today that we're running out of time," said Sen. Jon Kyl (R-Ariz.).
Kennedy's remarks to reporters, as well as an announcement by Senate Majority Leader Bill Frist (R-Tenn.) that the Senate would adjourn for the year by Nov. 21, were widely viewed as the beginning of the Medicare legislation end-game.
After more than three months of talks, negotiators have not reached accord on the four major differences between the House and Senate bills.
Until Tuesday, Kennedy was insisting that failure to produce a compromise bill was "not acceptable" because, given the growing federal deficit, it would be hard for Congress to agree again to spend $400 billion over 10 years on a prescription drug benefit.
"I hope that's still the case," Kennedy said, but "clearly, now it's in a freefall."
Kennedy "ought to hold his power until we make some decisions," said Sen. Charles E. Grassley (R-Iowa), vice chairman of the conference committee.
The biggest issue separating Republicans and Democrats is a provision in the House bill that would require traditional Medicare to begin competing against private managed-care plans, starting in 2010.
"This is a nonstarter, and it has been from the beginning," Kennedy said. "The Senate will not accept a proposal that's going to undermine the Medicare system, and that's what the conference is headed to."
Negotiators also remain divided over GOP plans to control Medicare spending and create a new tax shelter for medical expenses, as well as largely bipartisan proposals from both houses to allow the importation of U.S.-made drugs from Canada.