Senate Democratic leaders struggled Monday to get President Barack Obama's health care overhaul on track for passage by Christmas, faced with moderates' opposition to expanding Medicare and internal party disputes over abortion and importing cheaper prescription drugs.

In a make-or-break week, the leadership called an early evening meeting of all Democrats Monday, just hours after the latest obstacle emerged — Independent Sen. Joe Lieberman's threat to join Republicans in blocking the bill over a plan to allow uninsured individuals as young as 55 to purchase Medicare coverage.

The senator said Sunday he opposes it, speaking in a television interview and later in a private meeting with Majority Leader Harry Reid, D-Nev.

Separately, Sen. Bob Casey, D-Pa., a leading foe of abortion rights, was scheduled to meet with Obama at the White House to discuss the impasse over stiffening the bill's abortion restrictions.

Lieberman strongly rebutted charges that he flip-flopped to oppose the expansion of Medicare as part of health care legislation.

"Contrary to the claims of anonymous aides, Senator Lieberman told (Majority Leader Harry) Reid on Friday that he had problems with the Medicare provision," said Marshall Wittman, a spokesman for the Connecticut Independent.

"This position was also told to negotiators earlier in the week. Consequently, Senator Lieberman's position came as no surprise to the Democratic leadership. Any contrary charge by aides who cowardly seek to hide under the cloak of anonymity is false and self-serving," he added.

Wittman made his comments one day after Senate Democratic aides, speaking anonymously, said Lieberman had initially indicated support for a proposal allowing uninsured individuals as young as 55 to purchase Medicare coverage.

Democrats need 60 votes to overcome Republican opposition to the legislation, and have been counting on Lieberman to side with them.

Other issues stand in the way of the bill's passage as Democrats maneuver against unstinting opposition from Republicans.

Sen. Ben Nelson, D-Neb., has said he wants stricter limits on abortion. Casey has been working privately with Nelson in hopes of reaching a compromise on the issue.

Separately, Sen. Byron Dorgan, D-N.D.. is seeking a vote on his proposal to permit the importation of prescription drugs from Canada and other countries as a cost-saving measure for consumers. The proposal is opposed by the pharmaceutical industry and has drawn a noticeable lack of enthusiasm from the White House, which has been working closely with drugmakers in the months-long effort to enact health care legislation.

The developments came at the beginning of a week in which the White House and Reid had hoped to place the health care bill on a track for passage by Christmas. The measure would expand health coverage to millions who lack it and ban insurance industry practices such as denying coverage on the basis of pre-existing medical conditions.

Obama has also said he wants the legislation to slow the growth in medical spending nationwide. A top administration economic adviser acknowledged Monday that the Democratic-backed health care measure would raise spending in the short run, but would eventually generate more than enough savings to offset the expense of expanded coverage.

"Our bottom line is that the bills as they are coming through will genuinely slow the growth of health care spending, both public and private, by about 1 percentage point a year for an extended period," said Christina Romer, chair of the Council of Economic Advisers in a call with reporters.

Reid has been awaiting final cost estimates from the Congressional Budget Office on an as-yet-unreleased package of final revisions in the measure. The majority leader announced with fanfare last Tuesday night that liberals and moderates within his party had reached a compromise framework on the controversial issue of the government's role in the insurance industry.

It calls for jettisoning a plan for government-run insurance that Reid had initially placed in the measure. In its place would be the expansion of Medicare, alongside a system of nationwide plans to be offered by private insurers and overseen by the federal agency that supervises the health care system that members of Congress use. The government would set up its own plan if no private firms stepped forward, but Lieberman and others object to that and it is not expected to survive.