Comprehensive health care reform is teetering on the edge.

The House Democratic whip operation already may need to flip a dozen votes on health care reform into the “yes— column to pass a compromise overhaul, and all bets are off if Senate Democrats lose Tuesday’s Massachusetts special election and shatter their 60-vote supermajority.

President Barack Obama and Congressional Democratic leaders negotiated long into the night last week to hammer out key pieces of the reconciled House-Senate health care package, clearing the most obvious hurdle by reaching a deal with union leaders to pare back a “Cadillac— tax on high-cost health plans. But attention quickly shifted to Massachusetts Attorney General Martha Coakley’s (D) cratering Senate campaign, with multiple polls over the weekend showing her slipping further behind Republican state Sen. Scott Brown in deep blue Massachusetts.

Democratic leaders publicly espoused confidence that Coakley would eke out a victory, but behind the scenes were nearing full-out panic mode. Members were cutting checks to Coakley, Obama jetted to Massachusetts on Sunday to campaign, and House lawmakers like Rep. Barney Frank (D-Mass.) told reporters that a Coakley loss would kill the health care bill.

Even if Frank is wrong, Democrats face a serious climb if Coakley loses in her bid to succeed the late Sen. Edward Kennedy (D-Mass.), the veteran lawmaker who on his death bed called health care “the cause of my life.—

If Brown wins decisively, Massachusetts election officials would likely have to certify him the winner by early February — a time frame that would give House and Senate leaders only about two weeks to pass the health package. That’s a tall order given the need for a Congressional Budget Office cost estimate, House leaders’ vow to allow Members to review the bill for 72 hours before voting, the lack of a final deal yet on the thorny issues of abortion and immigration, and Senate procedural roadblocks that could add at least another three or four days of delay.

And the political backlash of jamming through the massive bill in the face of a defeat in what was thought to be a solid Democratic seat could make some Democrats balk.

Senate Democratic aides speculated that vulnerable incumbents, such as Sen. Blanche Lincoln (D-Ark.), could rethink their support for health care reform in that scenario. Additionally, Senate Majority Leader Harry Reid (D-Nev.), who is perhaps the most endangered of the chamber’s Democrats this year, is already facing considerable unrest back home for his role in pushing health care.

It’s a similar dynamic in the House, where leaders’ endgame relies on converting about a dozen moderate Democrats who voted against the bill two months ago. That task could be complicated even by a narrow Coakley win, if it’s interpreted by vulnerable Democratic incumbents as a sign of waning national support for the health care reform effort.

Some also have floated behind the scenes the prospect of the House simply passing the Senate bill, an idea rejected out-of-hand by Speaker Nancy Pelosi (D-Calif.) and other House leaders, who have talked about using the budget reconciliation process to push through a measure instead. Reconciliation would allow the Senate to pass at least part of the package with a simple majority and avert a GOP-led filibuster.

Many House Democrats have warned that they would not vote for the Senate-passed bill, including liberals unhappy with the package’s affordability levels for the middle class and the “Cadillac— tax on insurance plans that would affect millions of middle-class workers.

But a stinging defeat in Massachusetts Tuesday could change the dynamic, with liberals seeing their decades-long struggle for sweeping health care reform potentially slipping away and moderate Democrats fearing the outcome portends a wholesale slaughter across the country in November.

A Senate Democratic aide said nobody is expecting Pelosi will agree to take the Senate bill.

“Maybe in the aftermath of a loss in Massachusetts, that calculus will change, but it’s impossible to predict how a loss will affect the climate in the House,— said a Senate Democratic aide.

Another theoretical possibility would involve passing the Senate bill alongside a companion bill through reconciliation.

But using reconciliation would be messy, which is why Senate Democratic leaders and Obama have been loath to use it. For starters, it gives Republicans the ability to require votes on hundreds of politically charged amendments on the Senate floor without debate. And there are limitations on what can be accomplished. Unless 60 Senators vote to waive the rules, the reconciled bill can only deal with issues that affect the budget. Democrats would have the ability to include the “Cadillac— tax compromise, but their hands could be tied when it comes to language on abortion, for example, or on removing antitrust protections for insurance companies.

Democrats could also make another run at winning moderate Sen. Olympia Snowe’s (R-Maine) vote, appealing to her sense of history, but restarting those overtures would be awkward to say the least after Reid dismissed his negotiations with her as “a waste of time— in a recent New York Times interview.

It’s also not clear how Snowe’s involvement could make life any easier for Democrats, given that on some issues she is likely to pull the bill further to the right, and away from the more liberal House position. And her plan for a public insurance option trigger had garnered a filibuster threat from Sen. Joe Lieberman (ID-Conn.).

Yet giving up isn’t an option either, according to several Democratic aides. “It’s a lot easier to run with something than to run and say you couldn’t get it done even when you had a large majority,— said a House Democratic aide.

House Democratic leaders are likely to discuss the situation at their leadership meeting Tuesday afternoon, aides said.

Abortion, meanwhile, has shadowed the health care bill for months, with Democratic factions sniping at each other after Rep. Bart Stupak (D-Mich.) and a band of abortion opponents forced Pelosi to allow his amendment strictly forbidding women from buying insurance plans covering the procedure if they receive federal subsidies.

House and Senate negotiators have yet to resolve the abortion issue, but Pelosi has said she wants to strip out the Stupak language, which could cost her about 10 votes, according to Stupak.

Members of leadership reached out to the Michigan Democrat over the weekend, according to a Democratic aide — a sign of the difficulty they face in getting the votes for the bill. But cutting a deal with Stupak could alienate supporters of abortion rights who have vowed to oppose any bill with Stupak-like language included.

The $1.2 trillion House bill passed on a 220-215 vote with Stupak’s support. But Democrats have already lost a vote with the resignation of former Rep. Robert Wexler (D-Fla.). And moderating the Stupak language would likely mean losing the support of the sole Republican to vote for the package, Rep. Anh “Joseph— Cao (La.), leaving the majority no margin at all. Democrats are also concerned they could lose a couple of votes because any bill will not include a public insurance option, which numerous liberals threatened had to be included to get their votes, and because possibly a few Members of the Congressional Hispanic Caucus are angry at a ban on illegal immigrants buying insurance plans with their own money through new exchanges.

Just two members of the Progressive Caucus voted against the House bill — Reps. Eric Massa (D-N.Y.) and Dennis Kucinich (D-Ohio). Massa issued a statement last month titled “Still can’t vote for it,— calling the lack of a public option a “deathblow.—

Leaders would have to make up the difference by winning over support largely from a group of fiscally conservative Blue Dog Democrats who voted against the bill the first time out. Many of them, such as sophomore Rep. Jason Altmire (D-Pa.) or retiring lawmakers like Bart Gordon (D-Tenn.), have said in recent weeks that they prefer aspects of the Senate bill and will consider whatever measure ends up coming to the floor, but they have not committed to voting for anything.

Some told leadership that they wanted to support a final health care bill, but have continued to face enormous pressure back in their districts to oppose it. Nevertheless, senior Democratic aides believe they will be able to find the votes they need among this group, in part by appealing to the call of history. “Every person in the Democratic Caucus got into politics to deliver health care to people,— one said. “They may represent districts where it’s harder to defend, but when the chips are down and this is the vote that you’re going to be remembered for, I think a lot of people are going to find a way to get to ‘Yes.’—