Democratic Senators became increasingly intransigent over the weekend about what they can accept as part of a health care reform bill, further complicating Majority Leader Harry Reid’s (D-Nev.) goal of passing legislation by Christmas.

Moderate Senators are proving to be Reid’s latest headache, with several on drawing sharper lines in the sand Sunday on the $848 billion package.

Most surprising to Reid was Sen. Joe Lieberman’s (ID-Conn.) second filibuster threat in two months. After indicating as much on CBS’ “Face the Nation— on Sunday, Lieberman formally notified Reid that afternoon that he would oppose any bill that expanded Medicare.

Lieberman’s new threat appeared to be a reversal of his private assurances to Reid just a week ago. One Senate Democratic leadership aide said Reid was angry about the self-described Independent Democrat’s stance because Lieberman was one of the first Senators the Majority Leader spoke to about the proposal – which was devised by a Democratic group of five liberals and five centrists – and that Lieberman initially “voiced support— for the plan.

Publicly, Lieberman said last week he was “encouraged— by the groups’ proposals.

Meanwhile on Sunday, Sen. Claire McCaskill (D-Mo.) insisted that the Congressional Budget Office offer up a positive analysis of a possible public insurance option compromise, while Sen. Ben Nelson (D-Neb.) used his strongest language yet in a Washington Post article on how far the bill should go in restricting access to abortion.

Reid’s package hasn’t moved an inch since last week as he awaits the CBO score on the possible public option deal. And it seems, the longer the bill idles, the more problems Reid faces, whether on abortion, prescription drug reimportation or the burden that reform could place on the federal budget.

Lieberman, speaking on CBS’ “Face the Nation,— said, “We don’t need to keep adding on to the back of this horse because we’re going to break the horse’s back and get nothing done.—

At press time, Reid and Minority Leader Mitch McConnell (R-Ky.) were still trying to negotiate a deal on amendments to the health care bill. And Reid is hoping a favorable CBO cost estimate on the public option alternative will unify his Conference and allow him to close out the amendment process and move toward final passage of the bill. A CBO score on the proposed compromise, negotiated by a group of 10 liberal and moderate Democrats, could be available Monday.

The Democrats have kept the details of the public option compromise under wraps pending the release of the CBO score. Democratic leaders are hoping the proposal will quell the concerns of moderates like Lieberman without losing the support of liberals like Sanders, an Independent from Vermont.

A senior Democratic Senate aide said that in order to do that, the measure must provide affordable coverage, choice and competition, but in a cost effective way. The aide predicted the plan would receive broad Democratic support if it met those criteria, despite the initial unease from Senators about the so-called compromise.

Notably, this aide said, “There’s definitely a chance there will be more amendments while we’re waiting for the manager’s package,— suggesting that votes on additional proposals are not guaranteed and that the Democratic leadership is preparing to move to end debate on health care reform after the remaining few internal disagreements are resolved. Reid plans to use the manager’s amendment to package several Democratic proposals to cut down on the bill’s debate time.

“I hope we’re going to have a good score back from CBO, which will enable our leader to lay down the bill — the manager’s package — I hope by early [this] week,— Health, Education, Labor and Pensions Chairman Tom Harkin (D-Iowa) said.

Harkin and Democratic Conference Vice Chairman Charles Schumer (N.Y.) both participated in the negotiations on the public option compromise. The talks produced a proposal that would replace the current public option, which calls for the creation of a national, government-run insurance company with an opt-out provision for the states, with a measure that would grow current federal health care programs. It would expand Medicaid and allow individuals ages 55 to 64 to buy into Medicare.

The Senate was in session over the weekend to consider an omnibus appropriations bill. Although Members used the open floor time to debate health care reform and continue private meetings on the issue, no votes were held on amendments to the bill. The Senate voted just twice — on Saturday to end debate on the $446.8 billion omnibus and again on Sunday on final passage of the package.

Even as the Democrats hold their breath for the CBO score, abortion has emerged as another issue that threatens to prevent passage of the bill. Republicans are nearly unanimous in their opposition to the package, meaning Reid has little room for error within his own, 60-seat majority.

Nelson has problems with the public option as well as the abortion issue. The Nebraska Democrat has been adamant that he will not vote to end debate on the health care bill unless it explicitly prohibits federal health insurance subsidies called for in the legislation from being used to cover abortion.

Nelson’s amendment to address the issue was defeated last week. His proposal was modeled after a provision in the House bill sponsored by Rep. Bart Stupak (D-Mich.).

Sen. Bob Casey (D-Pa.), a co-sponsor of Nelson’s original amendment, is trying to fashion a compromise that would be acceptable to Democrats supportive of abortion rights.

“This is not one where you can split the difference,— Nelson told reporters Saturday, when asked about the prospects of compromising on the abortion language. “A lot of issues around here beg for compromise, whether it’s halfway there or two-thirds of the way there. This is different. I think that’s what makes it so challenging.—

Nelson said Casey is taking the lead on the abortion compromise because he is focused on the public insurance option deal. If an agreement on the abortion language can be reached, it would likely be inserted into the manager’s amendment. Casey on Saturday declined to comment on the status of his effort, including how it relates to his ability to support the final bill.

“All I can say is we’re working hard on it,— Casey said. “I’m trying to get this right.—

Meanwhile, Democrats are struggling with how to handle an amendment by Sen. Byron Dorgan (D-N.D.) that would authorize the reimportation of prescription drugs.

While the issue is unlikely to derail health care reform, it is indicative of the majority’s internal haggling over the many pieces of the package. Sen. Frank Lautenberg (D-N.J.) is offering a side-by-side amendment to the proposal that, if approved, would essentially negate the Dorgan amendment. The opposing party typically offers side-by-side amendments.

The Dorgan amendment appears to have bipartisan support, and if it passes, it could imperil a separate deal that Finance Chairman Max Baucus (D-Mont.) and the Obama administration made with the pharmaceutical industry to help pay for the health care bill.

Dorgan said over the weekend that his amendment would “strengthen— the existing bill, but Senate Democratic leaders appear to feel differently. Baucus on Saturday said resolution of this issue is “very important.—

Republicans, looking to further their case against the bill, used the weekend session to highlight the Democratic divisions. McConnell, in a press conference, lamented the fact that the Senate was diverted away from the health care reform debate to work on the omnibus.

“They can’t close down the bill. As you know, they’ve got serious problems,— McConnell argued. “The American people are almost screaming at us, ‘Please don’t pass this.’ They’ve got all kinds of internal problems. My assumption is, we’ll go back to the bill [today] and start voting on amendments. That’s what we should be doing; that’s what the American people expect us to be doing.

Emily Pierce contributed to this report.