The Senate hopes to decide on Saturday whether to move the bill to the floor for debate. The House of Representatives, meanwhile, has already passed its version of reform, leaving forward movement on overhauling the system solely in the hands of the Senate.

Failure to guide the measure to passage would represent a crippling, if not fatal, blow to a pillar of the basket of domestic reforms Obama promised in his presidential campaign.

While Democrats hold a significant majority in the Senate, its rules allow Republicans to block action through a delaying tactic known as a filibuster. To end a filibuster, the Democrats must assemble 60 votes. Currently, they hold 58 seats in the 100-seat upper chamber. Two independent members typically vote with the Democrats.

But the two independent votes are not assured, nor are those of three conservative Democrats representing states that normally vote Republican.

Majority Leader Harry Reid unveiled the long-awaited legislation Wednesday night to extend coverage to 30 million more Americans and force insurance companies to sell policies to all who want to buy them. Currently, the insurance industry regularly refuses coverage to people who already have a medical condition.

Obama and Democrats hailed the 10-year, $849 billion measure that would remake the U.S. health care system, relying on cuts in future Medicare spending to cover costs, as well as on higher payroll taxes for the well-off and a new levy on patients undergoing elective cosmetic surgery. Medicare is the government program that provides health insurance coverage for retirement-age Americans.

Aides to Reid said the bill would reduce deficits by $127 billion over a decade as estimated by the nonpartisan Congressional Budget Office. For the first time most Americans would be required to buy health insurance, and the bill would provide hundreds of billions of dollars in subsidies to help those with lower incomes afford coverage.

Employers would not be required to offer coverage, but medium and large companies would pay a fee if the government ends up subsidizing employees' insurance.

On a controversial issue that threatened to derail the House legislation, Reid's Senate measure would allow the new government insurance plan to cover abortions and would let companies that receive federal funds offer insurance plans that include abortion coverage.

A provision in the House bill — passed at the insistence of anti-abortion Democrats over strenuous objections from liberals — banned both those things. Reid failed in his attempt to include stricter abortion language.