WASHINGTON — In a show of unity, Senate Democrats sealed a 60-vote majority needed to advance health care legislation Saturday ahead of an evening showdown with Republicans eager to doom the bill and inflict a punishing defeat on President Barack Obama.

Two final holdouts, Sens. Mary Landrieu of Louisiana and Blanche Lincoln of Arkansas, 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 Thanksgiving.

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.

“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.”

Both 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, D-Nev., 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.

The legislation would require most Americans to carry insurance, and large firms would incur large costs if they did not provide it to their workforce.

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.

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, R-Mo., likening the bill’s supporters to the imprisoned investor who fleeced millions.

Sen. Judd Gregg, R-N.H., 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 of Kentucky. “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.”

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. One possibility would require the federal government stay out of the insurance business unless there was a shortage of competition or affordable coverage offered by private companies.

At its core, the legislation would create insurance exchanges beginning in 2014 where individuals, most of them lower income and uninsured, would shop for coverage. The bill sets aside hundreds of billions of dollars in tax credits to help those earning up to 400 percent of poverty, $88,200 for a family of four.

Additional funds would be available to help small businesses defray the cost of providing coverage to their employees.

The insurance industry would come under significant new regulation under the bill, which would first ease and then ban the practice of denying coverage on the basis of pre-existing medical conditions. Beginning in 2014, there would be no limits on lifetime coverage. Effective immediately, children could remain on their parents’ insurance policies until age 26, three years longer than under current law, another attempt to cut into the ranks of the uninsured.

Individuals would be required to purchase coverage or pay a fine, unless affordable coverage was not available. Larger employers would not be required to provide coverage, but would face penalties if they did not and any of their workers received federal subsidies to buy individual coverage.

To finance the expanded coverage, Reid proposed higher taxes as well as cuts totaling hundreds of billions of dollars in projected Medicare payments. Hardest hit would be the private insurance Medicare plans, although providers such as home health agencies would also receive significantly less in future years than now estimated.

The bill raises payroll taxes on incomes over $200,000 for individuals and $250,000 for couples. Reid eased the impact of an earlier proposal to tax high-value insurance plans, which has emerged as one of the principal methods for restraining the growth in health costs.

The bill includes tax increases on insurance companies, medical device makers, patients electing to undergo cosmetic surgery and drugmakers.

———

Associated Press writer Donna Cassata contributed to this article.