WASHINGTON (AP) – His calls for compromise rebuffed by Republicans, the Democratic chairman of the Senate Finance Committee revealed sweeping legislation Wednesday to remake the nation’s costly health care system largely along the lines outlined by President Barack Obama.

Sen. Max Baucus’ proposal, months in the making, drew quick criticism from liberals, who said his vision was too cramped, and from Republicans, who deemed it overly expansive. Yet whatever its fate, its mere release marked a critical turning point in Congress’ long and tumultuous debate over Obama’s top domestic priority.

The bill would make major changes to the $2.5 trillion U.S. health care system, including a requirement that all individuals buy health insurance or pay a fine. Language in the bill also prohibits insurance company practices like charging more to people with more serious health problems.

The bill would extend insurance to about 29 million Americans who now lack it. Most of the features in the measure would not take effect until 2013, after the next presidential election.

Despite the lack of Republican support, Baucus said he is holding out hope that some will support the plan when his committee votes on the bill next week.

Baucus’ proposal has been anticipated eagerly because, of several bills being considered in Congress, it is seen as the only one that could get support from Republicans as well as Democrats.

Obama had campaigned on a pledge of bipartisanship and has said he wants to reach a compromise with Republicans on a health care package. Liberal Democrats, however, doubt that Republicans can be persuaded to support any agreement and want Obama to use his party’s strong majorities in both chambers to push through legislation.

It remains unclear whether he has the votes to do that. Democrats are now one vote shy of the 60 votes they need to block Republican efforts to derail the bill in the Senate. Also, with congressional elections approaching next year, some Democrats from conservative states are wary about supporting an overhaul that voters may see as too liberal.

That means Baucus’ plan may offer Obama his best hope for getting health care legislation passed. The final version of the bill, if one is passed, could be shaped more by Baucus’ plan than by the proposals of any other lawmaker.

When the Finance Committee meets next week to vote on the plan, Majority Leader Harry Reid intends to begin debate on the Senate floor as early as late this month or early October. Across the Capitol, Speaker Nancy Pelosi, also a Democrat, has been waiting to see Baucus’ health care prescription before advancing companion legislation toward a vote by the House of Representatives.

“We cannot let this opportunity pass,” Baucus said as he outlined an $856 billion plan designed to protect millions who have unreliable insurance or no coverage at all, all the while restraining the explosive growth of medical costs.

The effect on the federal budget of one of the major concessions Baucus made in a so-far-unsuccessful search for Republican support – allowing cooperatives, rather than the federal government, to sell insurance in competition with private industry – was judged harshly.

Supporters claim the co-ops would compete effectively with private companies and help hold down the cost of insurance, but the Congressional Budget Office has made an unfavorable assessment of the idea, which could re-energize advocates of direct government competition.

Senate Republican leader Mitch McConnell of Kentucky, who has labored to keep his rank and file united in opposition, called it a partisan proposal that “cuts Medicare by nearly a half-trillion dollars and puts massive new tax burdens on families and small businesses, to create yet another thousand-page, trillion-dollar government program. Only in Washington would anyone think that makes sense, especially in this economy.” Medicare is the government’s main medical program; it mainly provides health care for the elderly.

Baucus’ legislation reflected nearly a year of preparation by the 67-year-old Montanan, a partially successful attempt to gain support from outside interest groups, and months of painstaking private negotiations with two other Democrats and three Republicans on the Finance Committee.

With White House backing, the nation’s drug makers and hospitals have agreed to defray part of the expense of the eventual bill, and the Pharmaceutical Research and Manufacturers of America is underwriting a television advertising campaign at a cost of tens of millions in favor of health care legislation. The AARP, a huge national influence group for people 50 years old and older, generally supports the effort as well, despite the cuts in planned Medicare spending. Even opposition from the insurance industry has been somewhat muted.

Even so, the private negotiations involving Republican Sens. Chuck Grassley of Iowa, Mike Enzi of Wyoming and Olympia Snowe of Maine came up short, at least for the time being.

Baucus told reporters he expected he would gain bipartisan backing before the bill emerges from committee, probably next week, an evident reference to Snowe. “This is a first step in the process,” Snowe said in a written statement, and she promised to continue to work with Baucus and Democrats on drafting a bipartisan bill.

The other Democrats in the talks were Kent Conrad of North Dakota and Jeff Bingaman of New Mexico.

Senate Finance Committee Chairman Sen. Max Baucus, D-Mont. gestures during his news conference on health care legislation, Wednesday, Sept. 16, 2009, Capitol Hill in Washington.