WASHINGTON -- Moderate Democratic senators remained reluctant yesterday -- and in one case, defiant -- about backing the government-run "public option" health care plan that party leaders are offering as a compromise, making it highly uncertain whether the plan can become law.

Independent Connecticut Sen. Joseph Lieberman, who caucuses with Democrats and was the party's vice-presidential nominee in 2000, said he'd back a filibuster to prevent a public option from coming to a final vote. "If the bill stays as it is now, I will vote against cloture, which is to say against the bill coming to final passage," he said.

The centrists, a loose-knit group of as many as 12 Democratic senators, are crucial to the success of any health care bill, because it takes 60 votes in the 100-member Senate to stave off a filibuster -- an extended debate to prevent a decisive vote -- and Democrats control 60 seats.

After Democrats met privately for about an hour yesterday, the moderates were largely unenthusiastic about Monday's proposal from Senate Majority Leader Harry Reid, D-Nev., to include a public option in the pending health care overhaul legislation, while allowing states to opt out of it.

While the Democrats, including Mr. Lieberman, are expected to vote with their party leadership at least to allow debate to begin, there are serious questions about whether they'll provide the votes needed to end debate over specific parts of the bill or, in the end, to approve the legislation.

Mr. Reid said he was unconcerned, calling Mr. Lieberman "the least of Harry Reid's problems."

Moderate Democratic senators long have voiced reservations about the public option and the cost of overhauling the health care system. Their roster includes Arkansas' Blanche Lincoln and Mark Pryor, Louisiana's Mary Landrieu, Virginia's Jim Webb and Mark Warner, Montana's Jon Tester, Nebraska's Ben Nelson, Indiana's Evan Bayh, North Dakota's Kent Conrad and Byron Dorgan, and Delaware's Thomas Carper.

Many said yesterday that they weren't certain what they might do. "I don't think we can reach any conclusions about this until we see the whole package," Mr. Conrad said. Mr. Pryor echoed, "I'm making no commitments until I see the bill."

While a majority of Senate Democrats are thought to favor a government-run system as one option in the health care legislation, the moderates would prefer a more modest approach, and some have said they flatly oppose Mr. Reid's plan.

Mr. Lieberman said the opt-out plan "creates a whole new government entitlement program." And Ms. Lincoln has said she'd oppose any new government-run health care system.

In addition, the two Maine Republican moderates who sometimes side with Democrats on key votes said they wouldn't cross party lines this time. "I don't see opt-out as any kind of compromise at all," said Sen. Susan Collins, echoing the position of Sen. Olympia Snowe.

Other key problems that Democratic centrists cited were the potential cost of the bill, and its lack of specifics. "I want to read the details first," Mr. Bayh said.

Mr. Reid offered few specifics when he met with his caucus, according to three senators who attended. Asked whether he learned more about the Reid plan's details, Mr. Nelson said "no." He and Mr. Bayh were among those who wanted to learn more about the plan's cost.

The legislation won't reach the Senate floor until the nonpartisan Congressional Budget Office analyzes its financial impact. President Barack Obama has said he wants a bill that won't increase the federal budget deficit over 10 years.

"If cost was no concern, I could vote for this independently," Mr. Bayh said, "but that's not the case."

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