Two final holdouts, moderate Democratic Sens. Mary Landrieu and Blanche Lincoln, announced in speeches a few hours apart on the Senate floor they would vote to clear the way for what is expected to be a bruising, full-scale health care debate after the Thanksgiving holiday next Thursday.

"The country suffers when there is a failure to act on serious challenges that millions of ordinary Americans face in their daily lives," Democratic Sen. Patrick Leahy said during the rare weekend session.

At a 10-year cost approaching $1 trillion, the measure is designed to extend coverage to roughly 31 million who lack it, crack down on insurance company practices that deny benefits and curtail the growth of spending on medical care nationally.

Almost everyone would be required to purchase insurance, and billions in new taxes would be levied on insurers and high-income Americans to help extend coverage.

No other developed country lacks a comprehensive health care program for its people, leaving nearly 50 million uninsured. Obama campaigned on the promise to change that. As president, he has made overhauling the health care system his top domestic priority.

Landrieu and Lincoln stressed they were not committing in advance to vote for the bill that ultimately emerges from next month's debate. Even so, their announcements marked a major victory for Senate Majority Leader Harry Reid, a Democrat, and the White House in a year-end drive to enact the most sweeping changes to the nation's health care system in a half-century or more.

"It is clear to me that doing nothing is not an option," said Landrieu, who noted the legislation includes $100 million to help her state pay the costs of health care for the poor.

Lincoln, who faces a tough re-election next year, said the evening vote will "mark the beginning of consideration of this bill by the U.S. Senate, not the end."

Congressional budget analysts put the legislation's cost at $979 billion over a decade and said it would reduce deficits over the same period while extending coverage to 94 percent of the eligible population.

The House approved its version of the bill earlier this month on a near party line vote of 220-215.

Democrats need 60 votes in the 100-member chamber to allow their health care bill to be debated by the full Senate. With opposition Republicans opposing the plan, they have to count on the votes of all 58 Democrats, plus two independents that generally vote with them.

After the vote Saturday night, senators will leave for a Thanksgiving holiday recess. Upon their return, assuming Democrats prevail on Saturday's vote, they will launch into weeks or more of unpredictable debate on the health care bill, with numerous amendments expected from both sides of the aisle and more 60-vote hurdles along the way.

A defeat Saturday might not definitively end hopes for a health care bill, but it would cast doubt on whether Democrats can cobble together any plan that the party could rally behind.

A Democratic victory, though, hardly guarantees that Obama's plan will pass. Even if a bill passes the Senate, it must be reconciled with the version narrowly approved by the House of Representatives. If a compromise can be reached, it would then have to be approved by both chambers.

In hours of debate before the Saturday evening vote, Republicans attacked the legislation as a government takeover of health care and worse.

"Move over, Bernie Madoff. Tip your hat to a trillion-dollar scam," said Sen. Kit Bond, likening the bill's supporters to the imprisoned investor who fleeced millions.

Republican Sen. Judd Gregg said Reid had delayed implementation of many of the bill's key provisions and made it look less costly as a result. He put the true price tag at $2.5 trillion over a decade once implemented.

"Senators who support this bill have a lot of explaining to do," said the Republican leader, Sen. Mitch McConnell. "Americans know that a vote to proceed on this bill is a vote for higher premiums, higher taxes and massive cuts to Medicare. That's a pretty hard thing to justify supporting." Medicare is the federal health care insurance program for the elderly.

That was a rebuttal to Landrieu and other Democrats who described the evening vote as one of procedure instead of substance.

In her remarks, Landrieu said, "I've decided that there are enough significant reforms and safeguards in this bill to move forward, but much more work needs to be done." She also touted the $100 million included in the legislation to help her state cover its costs under Medicaid, the state-federal health care program for the poor.

"I'm proud to have fought for it. And I will continue to," she said.

Reid worked for weeks drafting the legislation, a blend of bills approved earlier by two committees with new provisions designed to straddle the ideological divide among Senate Democrats.

Among the most controversial is a requirement for the government to sell insurance in competition with private industry, unless individual states opt out.

Landrieu, Lincoln and other Democrats have expressed unease about it, and attempts to modify the so-called public option are certain once debate begins in earnest.

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Associated Press writer Donna Cassata contributed to this article.