Updated: 3:30 p.m.

With the White House urging Senate Democratic leaders to cut a health care deal with wayward Sen. Joe Lieberman (ID-Conn.), Democratic sources said Monday they would use a special 5:30 p.m. caucus to determine whether the 59 other members of the Democratic Conference could acquiesce to his demands.

Lieberman told Senate Majority Leader Harry Reid (D-Nev.) on Sunday that he would filibuster any bill that included a proposal to allow those 55 to 64 to buy in to Medicare. That proposal is a key part of a tentative deal reached between 10 Democratic centrists and liberals last week, and sources said Lieberman initially did not raise any alarms about the proposal.

One senior Senate Democratic aide said Lieberman’s stance was of “increasing concern, but we’ll see what happens at caucus today.—

Several Senate sources said the White House has told Reid to try to cut a deal with Lieberman in order to pass the health care bill before Christmas, but it was unclear how that proposal might be greeted by other members of the Democratic Conference, given Lieberman has threatened to filibuster over two priorities for liberals — the Medicare buy-in and any public health insurance option. The Medicare buy-in provision was intended to satisfy Lieberman and other centrists concerned about the public option.

The White House disputed that they have instructed Reid to strike a deal with Lieberman.

“The report is inaccurate,— said White House spokesman Dan Pfeiffer. “The White House is not pushing Senator Reid in any direction. We are working hand in hand with the Senate Leadership to work through the various issues and pass health reform as soon as possible.—

The senior Senate aide said an unnamed Senator is likely to raise the issue of ditching the Medicare buy-in proposal at the Monday caucus and that the reaction of the rest of the Conference could dictate which direction Reid takes.

To pass a bill by Christmas, Reid will have to begin a time-consuming procedural march to overcome multiple GOP-led filibusters by the end of this week.

The Medicare buy-in proposal was included in a proposed compromise devised by five liberals and five centrists. Reid invited Lieberman to join the group, but the self-described Independent Democrat sent staff only. The Congressional Budget Office has yet to complete its work on a cost estimate of the deal, putting Reid in a bind just 10 days before the Senate is scheduled to adjourn for the year.

Senate Democrats held a special caucus last week to discuss the potential public option compromise, but few details were revealed to rank-and-file Members. The plan would drop the creation of a national, government-run insurance company in favor of a Medicare buy-in for individuals ages 55 to 64.

The deal would also expand Medicaid and create a national marketplace of private insurance plans managed by the federal Office of Personnel Management. Besides Lieberman, other moderate Democrats remain skeptical of the compromise.

Even though other Democrats are seeking changes to the legislation, Democratic leaders feel as if Lieberman’s threats are a more serious threat to passage.