With the House’s landmark vote complete, Senate Majority Leader Harry Reid is facing dissent in the Democratic ranks over his health-care strategy – leaving him struggling to meet a Christmas deadline and fielding White House pressure to get the bill done.

Even before Saturday’s House vote, senators had begun to question why Reid suddenly embraced a public health insurance option – one that he didn’t yet have the 60 votes to pass.

In the process, the Senate debate over health-care has stopped dead, raising the possibility the Senate won’t even begin floor debate until after Thanksgiving. Reid himself recently left open the chance the final bill could slip until early next year.

That remark earned him a visit from White House Chief of Staff Rahm Emanuel, who showed up in the majority leader’s office a day later to press him on the urgency of the Christmas deadline, according to two Senate aides.

But it’s not just timing. Reid’s first task is finding a way to bridge the divide in his caucus between liberals pushing for a public option and moderates who have resisted the most ambitious version of that plan.

And Senate moderates are clearly growing nervous about the process ahead — the difficulties of merging a still non-existent Senate bill with a more liberal bill from the House, one that has received the blessing of President Barack Obama.

In a private meeting last week with Finance Committee Chairman Max Baucus (D-Mont.), half a dozen moderate Democrats aired a long list of concerns about the differences between the two approaches: the $1.2 trillion price tag on the House bill, its reliance on a “millionaires tax” to fund the overhaul and the lack of common ground between the House and Senate on other taxes, among other issues.

The expectation has long been that the Senate’s approach would hold sway in a final bill, substituting a price-tag closer to $900 billion and eliminating the millionaires tax that’s a non-starter in the Senate. But Pelosi met one of her goals with the bill passed Saturday night, by bringing a more liberal version of the legislation into the conference in hopes of strengthening her hand in the negotiations. That could put additional pressure on the Senate to bend.

Among Senate moderates, the second-guessing began weeks ago. Baucus hosted a gathering Oct. 22, where he informed centrist senators about Reid’s decision to go with a public option that includes a chance for states to “opt-out” of the national plan.

“People were concerned, felt that that was probably the wrong approach,” said Sen. Ben Nelson (D-Neb.), who attended the meeting. “But, you know, we’ve elected the leader as our leader. If that’s what he chooses to do then we’ll obviously be, we’ll face it, and it may make the difference on whether we support [it].”

A series of closed-door merger meetings with the White House, Reid, Baucus and Sen. Chris Dodd (D-Conn.) resulted in Reid’s announcement two weeks ago that he was pursuing a public option.

The negotiations have since slowed precipitously as Reid waits for Congressional Budget Office estimates. Wavering Democrats haven’t received any hints of which direction Reid is heading. And Sen. Olympia Snowe (R-Maine), the one possible Republican vote, described the secrecy of the process as “a prescription for confusion and disaster.”

"It should have been far different than the way in which it's been handled. No one has ever understood this merger process," Snowe said. "Now we are facing the prospect of a unified bill that didn’t come out of either committee that, frankly, has been designed in the cloak of secrecy and darkness."

Some moderates are wondering if Reid should have simply put the $829 billion Finance Committee bill on the floor - and made only targeted additions and subtractions to avoid the kind of haggling with the CBO that is now holding up action.

Sen. Blanche Lincoln (D-Ark.), a key swing vote who faces a tough reelection next year, said “I don’t know why” Reid didn’t move forward with a bill that resembles the work of the Finance Committee. 

“I think it met all the criteria,” Lincoln said. “It was deficit neutral, which the president has been adamant about. We actually reduce the deficit in that bill. We created competition. We created the exchange pools. We created different options - more options for the public.”

As they’ve waited for Reid to produce his bill, Sen. Mary Landrieu (D-La.) said talks over strategy between rank-and-file members have intensified – and signaled that the moderates are not yet on board with Reid’s approach, which could seriously complicate Reid’s effort to get 60 votes for a bill.

“We want to go to the floor in the most united way possible,” Landrieu said, but didn’t seem to think Reid’s approach would do that.

Baucus sought, above all else, to deliver a bill to Reid that could win 60 votes on the Senate floor – and he pushed the majority leader to go with it. 

But Reid made “a judgment call” to go a different route, Baucus told POLITICO. Baucus is supportive of Reid and the process he chose, but suggested the delay in floor action has given ammunition to the GOP opposition.

“It’s a double edged sword,” Baucus said of the delay. “People have to become comfortable with it – and we also can’t take too much [time] because it frankly empowers those who are trying to kill it.”

Reid had no other choice, his spokesman Jim Manley said.

Two committees wrote legislation, and he could not simply disregard one of them, Manley said.

“He couldn’t discount the work of the HELP committee,” Manley said, referring to the Senate Health, Education, Labor and Pensions Committee, which also passed a health care bill. “The HELP committee worked hard to put together key legislation.”

The Congressional Budget Office isn’t expected to deliver cost estimates of the Senate bill until the end of this week. If senators aren't able to bring the bill to the floor until after Thanksgiving, they would have only four weeks to debate the most sweeping legislation in 40 years, vote on it, merge it with the House version, wait for another round of CBO estimates, and move it back through both chambers on final passage.

In other words, the feat is nearly impossible – by congressional standards.

If the Senate can complete work by Christmas, there is talk of keeping the House-Senate conferees in Washington through the holiday to negotiate the conference report.

Senate Majority Whip Dick Durbin (D-Ill.) said the concerns over the delay should be directed at the CBO – not Reid.

Asked if making less drastic changes to the bill could be enough to bring it to the floor, Durbin said: “That’s not good enough, saying we can get it through CBO in a hurry but can’t pass it on the floor.”

Indeed, Democratic leaders and top aides say that the Finance bill – which lacks a public option – wouldn’t have satisfied the vast majority of Democrats who want a government-run insurance option in the bill. One Democratic aide said there would be more disaffected liberals had Reid sided with Baucus and moved his bill to the floor.

In choosing the route with a public option, Reid sided with Sen. Chuck Schumer (D-N.Y.), who said “this was always going to be complicated” no matter which approach the leadership took.

“No one was going to be perfectly happy with anything done – stronger public option, no public option, no one was going to be completely happy,” Schumer said. “I think Leader Reid made the best decision given all the diverse views of the caucus.”

“This is how it all has to work,” Schumer said of the process.

Reid drew praise from Nancy-Ann DeParle, director of the White House Office for Health Reform, who said mergers can drag on for months.

“The leader did in less in 10 days” what can take much longer, DeParle told POLITICO in an interview Thursday.

But aides and senators said that the longer the bill is delayed, the harder it’s becoming to defend it.

Sen. Mark Warner (D-Va.), a moderate freshman, said he prefers the Finance bill over the other health care bills moving through Congress. But he dodged a question on whether he wanted Reid to take the Finance bill to the floor, joking that he had to “run to a secret meeting.”

“It’s just going to be a starting point for a lot of us who have not been on the committee to really weigh in,” Warner said of the bill eventually going to come to the floor.

But some say the battle between the moderates and the liberals is about to begin in earnest.

“I think a showdown is going to happen no matter what,” said Sen. Maria Cantwell (D-Wash.), who sits on the Finance Committee. “I’ll let the leader decide what’s the best way to have the showdown.”