Washington ? His calls for compromise rebuffed by Republicans, the Democratic chairman of the Senate Finance Committee unveiled 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 Finance Committee is to meet next week to vote on the plan, and after combining it with another panel’s bill, Majority Leader Harry Reid intends to begin debate on the Senate floor late this month or early October. Across the Capitol, Speaker Nancy Pelosi has been waiting to see Baucus’ health care prescription before advancing companion legislation toward a vote by the House.

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

Congressional budget experts estimated the proposal would reduce the ranks of the uninsured by 29 million over a decade. They also predicted the plan would trim federal deficits by $49 billion over the same period and suggested savings in the range of hundreds of billions of dollars might result for the decade that follows.

Many of the bill’s major provisions would be delayed until 2013, after the next presidential election.

But the impact of one of the key 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.

“They seem unlikely to establish a significant market presence in many areas of the country,” wrote Douglas W. Elmendorf, head of the Congressional Budget Office.

Supporters claim the co-ops would compete effectively with private companies and help hold down the cost of insurance, but CBO’s assessment is likely to re-energize advocates of direct government competition.

White House press secretary Robert Gibbs called the overall legislation an “important building block” that “gets us closer to comprehensive health care reform.”

Reid, too, described it as “another important piece to the puzzle” on the road to health care legislation. Pelosi said that while the bill would do less than House legislation to make coverage more affordable, its emergence “will move this historic debate forward.”

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.”