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.