WASHINGTON — In a surprise setback for Democratic leaders, Senator Joseph I. Lieberman, independent of Connecticut, said on Sunday that he would vote against the health care legislation in its current form.

The bill’s supporters had said earlier that they thought they had secured Mr. Lieberman’s agreement to go along with a compromise they worked out to overcome an impasse within the Democratic Party.

But on Sunday, Mr. Lieberman told the Senate majority leader, Harry Reid, to scrap the idea of expanding Medicare and abandon any new government insurance plan or lose his vote.

On a separate issue, Mr. Reid tried over the weekend to concoct a compromise on abortion that would induce Senator Ben Nelson, Democrat of Nebraska, to vote for the bill. Mr. Nelson opposes abortion. Any provision that satisfies him risks alienating supporters of abortion rights.

In interviews on the CBS News program “Face the Nation,” Mr. Lieberman and Mr. Nelson said the bill did not have the 60 votes it would need in the Senate.

Senate Democratic leaders, including Mr. Reid and Senator Charles E. Schumer of New York, said they had been mindful of Mr. Lieberman’s concerns in the last 10 days and were surprised when he assailed major provisions of the bill on television Sunday. He reiterated his objections in a private meeting with Mr. Reid.

A Senate Democratic aide, perplexed by Mr. Lieberman’s stance, said, “It was a total flip-flop, and leaves us in a predicament as to what to do.”

Democrats are desperately trying to round up 60 votes and conclude Senate debate on the health care bill before Christmas.

Mr. Reid could not immediately figure out how to achieve that goal at a meeting he held Sunday with senior Democratic senators and White House officials, including Mr. Obama’s chief of staff, Rahm Emanuel, according to Senate Democratic aides.

Marshall H. Wittmann, a spokesman for Mr. Lieberman, said the Connecticut senator “notified Senator Reid on Friday that he had severe misgivings about the Medicare buy-in proposal, so his comments on ‘Face the Nation’ should not have come as a surprise to the leadership.”

The Senate Republican leader, Mitch McConnell of Kentucky, said that passage of the bill was looking less and less inevitable. The Democrats “are in serious trouble on this,” he said, “and the core problem is the American people do not want us to pass it.”

On television Sunday, Mr. Lieberman said: “We’ve got to stop adding to the bill. We’ve got to start subtracting some controversial things. I think the only way to get this done before Christmas is to bring in some Republicans who are open-minded on this, like Olympia Snowe.”

Senator Snowe, of Maine, has tried to find common ground with Democrats, but has rejected Mr. Reid’s proposal to let uninsured people 55 to 64 years old purchase coverage under Medicare.

“You’ve got to take out the Medicare buy-in,” Mr. Lieberman said. “You’ve got to forget about the public option. You probably have to take out the Class Act, which was a whole new entitlement program that will, in future years, put us further into deficit.”

Class Act refers to a federal insurance program for long-term care, known as the Community Living Assistance Services and Supports Act.

Mr. Lieberman said he would have “a hard time” voting for a bill with the Medicare buy-in.

“It has some of the same infirmities that the public option did,” he said. “It will add taxpayer costs. It will add to the deficit. It’s unnecessary. The basic bill, which has a lot of good things in it, provides a generous new system of subsidies for people between ages 55 and 65, and choice and competition.”

Mr. Nelson said he wanted to know the cost of the Medicare buy-in. “I am concerned that it’s the forerunner of single payer, the ultimate single-payer plan, maybe even more directly than the public option,” he said.

Mr. Lieberman said: “The bill itself does a lot to bring 30 million people into the system. We don’t need to keep adding onto the back of this horse, or we’re going to break the horse’s back and get nothing done.”

Even if Senate Democratic leaders were prepared to meet Mr. Lieberman’s demands, they would still need to resolve intraparty disputes over insurance coverage for abortion.

Aides to Mr. Reid met Saturday with advocates of abortion rights to explore ideas for a compromise.

Details were sketchy. Under one idea, some health plans receiving federal subsidies could offer optional coverage for abortion, but they could not use federal money to pay for the procedure. They would have to use money taken from premiums paid by subscribers and would have to keep it separate from federal money.

Critics of abortion say such requirements for the segregation of funds are an accounting gimmick.

In hopes of placating opponents of abortion, Mr. Reid is also considering an increase in the federal tax credit for adoption of children and a new program to provide services to pregnant high school and college students.

Both ideas were proposed by Senator Bob Casey, Democrat of Pennsylvania, who opposes abortion but generally supports the overall bill.

“Many teens and women who face an unplanned pregnancy do so with little or no support,” Mr. Casey said.