Senate Democratic liberals and centrists have been working feverishly this weekend to craft a compromise on the public insurance option that can satisfy all factions of the party and secure a filibuster-proof vote on health care reform before Christmas.

Sen. Jay Rockefeller (D-W.Va.), one of the prominent liberals among the group of Members that has been meeting, said the goal is to complete negotiations by the end of the weekend. Referencing the next meeting at 4 p.m. Sunday, Rockefeller said, “We’re going to keep meeting. We’ll be meeting until midnight if it takes that.”

Senators emerged upbeat from a meeting of the two sides Saturday afternoon, saying they were moving forward, but provided few details about the proposals they may or may not be considering.

“We’re making good progress, and we are discussing the public option and related issues,— said Senate Democratic Conference Vice Chairman Charles Schumer (N.Y.), who noted they had met three times Saturday.

In addition to Rockefeller and Schumer, participants in the Democratic meetings Saturday included Health, Education, Labor and Pensions Chairman Tom Harkin (Iowa) and Sens. Sherrod Brown (Ohio), Russ Feingold (Wis.), Tom Carper (Del.), Ben Nelson (Neb.), Mark Pryor (Ark.), Blanche Lincoln (Ark.), and Mary Landrieu (La.). Schumer and Pryor have been leading the effort.

It was unclear why Sen. Joe Lieberman (ID-Conn.) has not been at the previous meetings of the group, considering he has threatened to filibuster any bill that includes a public option. However, Rockefeller said he is “on the list” of invitees.

Rockefeller said Majority Leader Harry Reid (D-Nev.) brought the group together to solve the party’s internal differences over the public option. Liberals want a government-run insurance plan, while centrists have been resistant to an option that would put the government in the driver’s seat on health care. Reid will likely need all 60 Members of the Senate Democratic Conference to beat back a GOP-led filibuster of the measure, given the unanimous Republican opposition.

“If I had to make any criticism I’d say, well, why weren’t we doing this— earlier? Rockefeller said. “Harry started this process. He just called me up and said there’s five of you progressives and then five moderates — people who might not vote for the bill, etc. — and you’re going to start meeting and we’ve been meeting.—

One of the reasons the group has been intent to finish its negotiations this weekend is because of the potential lag time between an agreement and a usable cost-estimate from the Congressional Budget Office, Senators acknowledged. Reid hopes to be able to start the clock ticking on time-consuming procedural maneuvers that will lead to final passage by the end of next week, aides and Senators said.

Still, Schumer indicated that CBO may not be the driver of when an agreement is announced.

“We’re going to try to get CBO to be as quick as possible,— Schumer said. “But, you know what? You can’t write a bill based on CBO. You have to write a bill based on what’s best for the country and then get CBO to score it.—

Of the secrecy surrounding the high-pressure negotiations, Rockefeller joked, “When [reporters] said Sen. Harkin said that [the Office of Personnel Management] was discussed, I denied even knowing who Sen. Harkin was.—

Rockefeller explained that he was unable to give many details on the substance of the talks.But he said he is optimistic that the group will be successful: “People are open in ways that have not been open before. … I have a good feeling about it.—