Senate Democrats made clear Monday they would pursue a go-it-alone strategy to enact health care reform this year, as Majority Leader Harry Reid (D-Nev.) announced that the final bill would include a public insurance option proposal favored by liberals and universally opposed by Republicans.

Reid said Senate negotiators and the White House agreed on a national public insurance option with an opt-out provision for states that do not want to participate. The measure — along with various proposals for other key portions of the final Senate bill — are now in the hands of the Congressional Budget Office for a cost estimate.

Leading backers of the public insurance option immediately offered their support for the proposal. Sen. Jay Rockefeller (D-W.Va.), who repeatedly has pushed back against a watered-down health care plan that would satisfy centrists, hailed the opt-out proposal as “strong— and uncompromising.

But moderate Sen. Olympia Snowe (R-Maine), President Barack Obama’s best hope for GOP support on his health overhaul, was critical.

“I am deeply disappointed with the Majority Leader’s decision to include a public option as the focus of the legislation,— said Snowe, who favored allowing a public option only as a last resort if private insurers don’t provide affordable coverage to all. “I still believe that a fallback, safety net plan, to be triggered and available immediately in states where insurance companies fail to offer plans that meet the standards of affordability, could have been the road toward achieving a broader bipartisan consensus in the Senate.—

The Democratic Conference was scheduled to be briefed on Reid’s health care package Tuesday. The Caucus meeting will be critical, particularly since some moderate Democrats are queasy about voting for a national public insurance option they fear might smack too much of a government takeover of health care.

But most moderates have over the past few weeks resigned themselves to voting for some form of a public option. And Democratic sources said Monday that only two Democrats, Sens. Mary Landrieu (La.) and Ben Nelson (Neb.), would not yet commit to moving ahead with a bill that includes a public option with opt-out.

“Most moderates are probably OK with it — or at least they wouldn’t hold up a bill because of it,— said one Democratic Senate aide who works for a moderate. “But a handful of moderates are very nervous about voting for a bill at this stage that includes a national plan of any kind.—

The public insurance option has been the most controversial aspect of the health care debate this year, particularly among Democrats. Moderates have been skittish, while liberals have pressed for a bold, government component akin to Medicare.

But in announcing to reporters his decision to move forward with a public insurance option with a state opt-out, Reid appeared to suggest that Democrats were done courting Republicans and resigned to the reality that major reform could only pass the Senate if all 60 Senators in the Democratic Conference came to an agreement.

Snowe was the lone Republican to back the Finance Committee’s version of a health reform bill earlier this month. That plan contained a proposal for a network of health insurance cooperatives rather than a public insurance option.

Had Reid tried to keep Snowe on board, he would likely have had to move a bill with the Maine Republican’s proposal for the trigger.

But that proposal may have cost him some Democrats.

Reid said there was agreement among the White House, Finance Chairman Max Baucus (D-Mont.) and Sen. Chris Dodd (D-Conn.), all of whom negotiated the final Senate plan, to go instead with the opt-out. The proposal is the brainchild of Democratic Sens. Charles Schumer (N.Y.) and Tom Carper (Del.), a key centrist. Carper originally proposed a public option with an opt-in clause; Schumer countered with the opt-out provision.

“I spent countless hours over the last few days in consultation with Senators who have shown a genuine desire to reform our health care system, and I believe there is strong consensus to move forward in this direction,— Reid said.

A senior Democratic Senate aide said they decided to go with the opt-out because Democrats generally favor it, and the few who do not like it “signaled— that it would not stop them from voting for health care reform legislation. Republicans accused Reid of playing to the party’s powerful liberal base and said it would incite a public backlash.

“His press conference [on Monday] is clearly an effort to keep the lifeblood of a deeply unpopular approach — even amongst Democrats — from drying up,— a senior Republican Senate aide said.

Emily Pierce contributed to this report.