WASHINGTON - Sen. Joe Lieberman (I., Conn.), whose vote is critical to the prospects of health-care legislation, threatened yesterday to join Republicans in opposing the bill if it includes a proposal to expand Medicare.

Lieberman expressed his opposition twice during the day - in an interview with CBS, and later, according to Democratic officials, in a private meeting with Senate Majority Leader Harry Reid.

Reid, who is hoping to pass the legislation by Christmas, needs 60 votes to overcome Republican objections, and has been counting on Lieberman to provide one.

But appearing on CBS, Lieberman said of the Medicare proposal, "Though I don't know exactly what's in it, from what I hear, I certainly would have a hard time voting for it because it has some of the same infirmities that the public option did."

The public option would have created a government-run health insurance plan to compete with private plans.

"It will add taxpayer costs. It will add to the deficit. It's unnecessary," he added of the Medicare provision that Reid last week hailed as part of a breakthrough between liberals and moderates.

Democrats who had favored that public option only grudgingly let it go, in return embracing the Medicare proposal as a way to help people 55 to 64 - a group vulnerable to losing employer-based health insurance when it is needed the most.

Democratic aides, speaking on condition of anonymity, said Lieberman later told Reid he would support a Republican-led filibuster against the bill if it contained the Medicare provision.

The same aides added that Lieberman had responded differently last week when Reid asked him privately about the proposed provision, which is backed by party liberals.

"He voiced support for the idea," said one official. Lieberman's public comments last week were also generally favorable. The aides spoke on condition of anonymity, saying they were not authorized to comment publicly.

If Lieberman follows through on his plans, he leaves Democrats with few options as they try to pass legislation before Christmas.

They could strip out the provisions he opposes, and hope liberals overcome their inevitable unhappiness and vote for the bill. For that approach to have any chance of success, Obama would have to lobby heavily in its favor.

The bill's supporters could turn to Republicans instead in search of support, but that is unlikely to produce a compromise in the next few days.

On CBS, Lieberman pleaded with Democrats to start subtracting from the overhaul what he called expensive proposals.

"We don't need to keep adding onto the back of this horse or we're going to break the horse's back and get nothing done," he said.

While Lieberman drew most of the attention, Democratic Sens. Claire McCaskill of Missouri and Ben Nelson of Nebraska also expressed concern about the proposal.

"I'm concerned that it's the forerunner of single-payer - the ultimate single-payer plan, maybe even more directly than the public option," Nelson said.

"The whole reason we're doing this bill is to bring down cost, first for the American people in health care, and secondly for the deficit," said McCaskill.

Asked if she would vote against the bill if it raised health-care costs overall, she said, "Absolutely."

Sen. John D. Rockefeller 4th (D., W.Va.) said he was working with Lieberman and others on controlling Medicare costs, and he voiced confidence the group would get past the divisions.

Lieberman, Nelson, and Rockefeller spoke on CBS's Face the Nation, McCaskill on Fox News Sunday.