Senate Democratic moderates warned Tuesday that they may block passage of a massive health care overhaul this year if Majority Leader Harry Reid (D-Nev.) insists on pushing reform legislation that contains a public insurance option.

Skeptical moderate Democrats are unlikely to block a motion to proceed to begin floor debate on Reid’s plan, sources say. But Reid could end up with a revolt on his hands once the amendment process is over, should the bill still include a public insurance option — even one with an opt-out provision for the states.

Sen. Joe Lieberman (ID-Conn.), who caucuses with the Democrats, threw down the gauntlet Tuesday, saying he would support a filibuster of the plan if — in the end — it includes a public insurance option of any kind. And centrist Democratic Sens. Evan Bayh (Ind.), Mary Landrieu (La.) and Ben Nelson (Neb.) were noncommittal, saying they need to see Reid’s bill before deciding on procedural votes.

“I’m very skeptical about what [Reid] outlined yesterday for a number of reasons — not only because of the concern with the public option,— Landrieu said. “But I am going to continue to work for a principled compromise. And that’s all I’m going to say.—

According to one aide to a moderate Democrat, the centrists are not “blustering.— This aide said the creation of a public insurance option is difficult for many moderate Senators to swallow.

“We may be able to get on the bill— and begin debate, the aide said. “But we are going to have a big problem getting off the bill with a national opt-out. It doesn’t work.—

Not all Democratic moderates have drifted away from leadership, however. Nelson said he could vote for a bill that includes a public insurance option, with Bayh focusing much of his criticism of the legislation on its costs and its potential impact on the federal deficit.

Reid’s early problem appears to be a lack of specifics. The Majority Leader, who negotiated the final blueprint with the White House and Finance Chairman Max Baucus (D-Mont.) and Sen. Chris Dodd (D-Conn.), has forwarded various proposals to the Congressional Budget Office to receive a cost estimate.

The final language of the bill won’t be decided upon until after the CBO responds. And the Democratic Conference is unlikely to be presented with the legislation’s details until that process is concluded.

“I do support the process. I think we need to keep this thing moving through the process, see how it develops over time,— moderate Sen. Mark Pryor (D-Ark.) said Tuesday. “I haven’t committed to [supporting procedural votes] — I won’t really do that until I have a chance to actually see the bill. But my inclination is to support the process.—

Pryor indicated he could support a public insurance option with an opt-out clause for the states. Conversely, Sen. Blanche Lincoln (D-Ark.), who could face a tough re-election battle next year, is signaling her opposition, and she has been noncommittal on whether she would vote with Democrats to end debate on the final measure and move to passage.

Reid unveiled the opt-out proposal on Monday, which he hoped would appease liberals but still allow Democratic moderates breathing room at home.

Republicans, including moderate Sen. Olympia Snowe (Maine), are unified in their opposition to Reid’s proposal and have problems with other aspects of the package, forcing Reid to try to win over every member of his diverse Conference of 60 Democrats.

[IMGCAP(1)]The Majority Leader is taking the long view, and he appears unconcerned by the early opposition from Democratic moderates, although he is not dismissing it. According to a senior Democratic Senate aide, Reid understands that some centrists might be playing to a home-state crowd, while others are looking for bargaining power as the final bill takes shape.

“Between now and the end of this, there’s going to be a lot of posturing,— this senior aide said. “But no one in this caucus wants to be responsible for bringing down health reform.—

“Why don’t we wait, take this one step at a time,— Reid said Tuesday when asked by reporters to comment on the moderate pushback. “We’ll get it on the floor; we’ll have an amendment process and that’s what we do.—

Even as Reid lobbies for support of the public insurance option, the cost of the bill looms as another possible hiccup.

Reid, Baucus and Dodd chose to forward a number of proposals to the CBO for cost estimates; Reid wants to put the most cost-effective options into the final bill while also keeping 60 of his Senators in the “yes— column.

Obama pledged in September that he would not sign a bill that exceeded $1 trillion over 10 years and added to the federal deficit, and for some moderates, that is more important than whether the measure contains a public insurance option.

“My broader concerns, as I’ve tried to indicate, are about whether it’s fiscally responsible or not and about the impact it’s going to have on ordinary people who currently have insurance,— Bayh said. “I want to read the bill.—