Updated: 7:36 p.m.Sen. Joe Lieberman (ID-Conn.) formally notified Senate Majority Leader Harry Reid (D-Nev.) on Sunday afternoon that he would filibuster the health care reform bill if it includes a Medicare “buy-in— provision.Lieberman’s position came as a surprise to Reid, considering the self-described Independent Democrat was among the first people Reid spoke to about the Medicare provision when it was discussed by a Democratic group of centrists and liberals attempting to craft a compromise that could secure the votes of all 60 Members of the Democratic Conference. At the time, Lieberman “voiced support— for the plan, according to a Senate Democratic leadership aide.Lieberman spokesman Marshall Wittmann strongly disputed the leadershipaide’s account. “The suggestion by an anonymous ‘aide’ that Senator Lieberman eversupported the Medicare buy in proposal is absolutely and totally false. The fact that the ‘aide’ won’t identify him/herself is a testimony that they are telling a deliberate falsehood or he/she is completely confused,— Wittmann said.One Lieberman aide said the Senator notified Reid on Friday that he had“problems— with the Medicare provisions. This is the second time that Lieberman has threatened to filibuster a final health care measure over a provision that is key to securing the votes of liberals. Lieberman first indicated his intentions on CBS’ “Face the Nation— program Sunday morning.Lieberman’s stance means that Reid either has to abandon a recently crafted compromise between liberals and centrists or will have to secure the vote of moderate Sen. Olympia Snowe (R-Maine) and possibly Sen. Susan Collins (R-Maine) in order to find the 60 votes that he needs to overcome a GOP-led filibuster. “It’s all coming down to one guy who’s prepared to vote against the interests of children and families in Connecticut who need health care reform,— said one Senate Democratic leadership aide, referring to Lieberman.The aide indicated that Reid was angry about the turn of events, considering Reid has essentially already agreed to eliminate the bill’s public health insurance option based on Lieberman and other centrists’ opposition. Lieberman was the most intransigent of the centrists on the question of the public option, having threatened nearly two months ago to filibuster if it was included.A week and a half ago, Reid convened a group of five liberals and five moderates — to which Lieberman was invited — to solve the concerns of centrists who were opposed to a public option. Lieberman declined to attend the meetings in person and sent his staff. The 10 Senators essentially agreed to drop the bill’s public option with an opt-out option for states, in part to explore a plan that would allow seniors ages 55 to 64 to buy into the Medicare program.Lieberman initially said he was “encouraged— by the tentative deal, which is still being reviewed by the Congressional Budget Office. An official cost estimate from the CBO is expected as early as Monday.Sen. Ben Nelson (D-Neb.) also indicated Sunday a growing uneasiness with the plan, even though he participated in the group of 10. But he said the provision could be axed if the CBO numbers indicate the plan is too costly or would undermine Medicare in some other way.“Some people may have to back away. They may like the idea, but they have to back away because the numbers don’t work,— Nelson told reporters Sunday in the Capitol.But Nelson was far from being as intransigent as Lieberman appeared to be.“I think those of us who’ve raised questions have said there’s still room for negotiation,— Nelson said.