With Republicans united in opposition, and conservative Democrats and the Senate's two independents expressing reservations, Majority Leader Harry Reid (D., Nev.) faced a daunting challenge in building the filibuster-proof majority needed for final passage.

He promised to keep the Senate working through weekends on the 2,074-page, $849 billion bill in hopes of bringing it to a vote by Christmas and landing a measure on President Obama's desk before the end of January.

And he sought to remind Democrats that, after investing so much time and political capital on the health-care issue, the price of failure could be high.

"While each of us may not say yes to each word in this bill as it currently reads, let us at least admit that simply saying no is not enough," Reid said.

The legislation marks the most ambitious effort in decades to provide near-universal health coverage, slap new regulations on insurance companies, and curb skyrocketing health costs.

Republicans have considerable power to extend the debate into the new year. Minority Leader Mitch McConnell (R., Ky.) has indicated he would like at least six weeks of discussion on the bill.

"Kentuckians want to know how spending trillions of dollars we don't have on a plan that raises health-insurance premiums and taxes on families and small businesses is good for health care or for jobs or for the economy, for that matter," McConnell said as he returned from a Thanksgiving trip to his home state.

But just as debate began, the nonpartisan Congressional Budget Office issued a report challenging Republican assertions that the bill would drive the cost of insurance through the roof.

The CBO, the arm of Congress that analyzes federal budget and spending proposals, found that under the Senate bill, premiums for most people - those in group plans provided by their employers - would remain unchanged or even drop.

In a sign of how high the stakes are for Obama, White House chief of staff Rahm Emanuel and other senior administration officials traveled to Capitol Hill yesterday to plot strategy with Senate leaders.

Democratic leaders acknowledge that major provisions may have to be changed to win the 60 votes needed to break an expected GOP filibuster and pass the bill.

Republicans began their attack with an amendment to strip out provisions that would curb the growth of Medicare spending by nearly $500 billion over 10 years.

Democrats said the savings would come from squeezing excessive subsidies and inefficient practices. Sen. John McCain (R., Ariz.) said they would curb benefits to the elderly and cater to special interests that lobbied the White House.

McCain also sought to tap into anxiety unleashed by a recent recommendation from a federal advisory panel that women in their 40s should no longer get annual mammograms. McCain argued it was a window onto the kind of government intervention into medical decisions that the bill would encourage.

Democrats' first amendment was designed to allay those concerns. The amendment, introduced by Sen. Barbara A. Mikulski (D., Md.), would guarantee women's access to preventive health care, including mammograms, that their doctors consider necessary.

The CBO report was produced at the request of Sen. Evan Bayh of Indiana and other wavering centrist Democrats whose support for the bill is crucial to passage.

Bayh said he was reassured by the report's conclusion that people insured through their employers - 83 percent of the nonelderly population with coverage - would see little or no difference in their premiums by 2016.

The report found that for the 17 percent who buy individual policies, premiums could rise 10 to 13 percent by 2016, mostly because the policies would provide more generous benefits than now. For half of those affected, the CBO said, their costs would go down because they would receive federal premium subsidies.