President Barack Obama's signature drive to remake US health care faced a critical Senate test vote Saturday, amid bitter 11th-hour debate and behind-the-scenes wrangling on a century-old policy feud.

Lawmakers were to vote at 8:00 pm (0100 GMT Sunday) on whether to formally take up White House-backed Democratic legislation estimated to extend coverage to some 31 million Americans who currently lack it.

The measure, which includes a government-backed insurance program to compete with private firms and restrictions on dropping care for pre-existing ailments, is estimated to cost 848 billion dollars through 2019 but cut the sky-high US budget deficit by 130 billion dollars over the same period.

If Democrats held together and rallied the 60 votes needed to ensure passage over united opposition from the 100-seat chamber's 40 Republicans, the Senate was due to take up the bill around November 30th for at least three weeks.

A successful vote after that would force the Senate and the House of Representatives, which passed its own version of the legislation November 7, to craft a compromise bill and vote again in order to send it to Obama.

Republicans, one of whom has vowed a "holy war" against the bill, hope to delay the battle into next year with the expectation that the 2010 midterm elections may make it harder for centrist Democrats to support the legislation.

As debate opened Saturday, Republican Minority Leader Mitch McConnell warned against passing "this staggering spending program at a time when many would argue our international bankers, the Chinese, are lecturing us about debt."

Democratic Majority Leader Harry Reid shot back that McConnell had unquestioningly backed spending hundreds of billions of dollars for the "war of choice" in Iraq under then-president George W. Bush.

Reid charged it was "beyond the pale" for McConnell "to lecture us now on debt when not only the war but the other actions of the Bush administration drove this country into deep debt."

The heated debate was unlikely to sway the most-watched wavering Democrats, Louisiana's Mary Landrieu and Blanche Lincoln of Arkansas, whose vote was perhaps most in doubt because she faces a tough reelection campaign in 2010. Women's health in political tug-of-war.

On Thursday, another swing-vote Democrat, Senator Ben Nelson of Nebraska, said he would vote with his party on Saturday while warning he might side with Republicans in subsequent fights.

The White House, which has wooed undecided Democrats, declared the legislation "a critical milestone" on Friday and warned "the nation cannot wait another year for health insurance reform."

After next week's Thanksgiving holiday break, Reid was speak to individual senators to "make sure they each have some peace of mind about what the bill does and can support it, and if they have a concern, address it," said the Democrats' chief Senate vote counter, Dick Durbin of Illinois.

The House of Representatives approved its own trillion-dollar version in a 220-215 squeaker only after winning over a platoon of centrist Democrats by toughening restrictions on federal funds subsidizing abortions.

The Senate version does not include that stricter language, and changes several other key provisions of what would be the most sweeping overhaul of its kind in four decades.

The United States is the world's richest nation but the only industrialized democracy that does not provide health care coverage to all of its citizens, with an estimated 36 million Americans uninsured.

Several US presidents since Teddy Roosevelt in the early 1900s have sought to overcome the traditional US suspicion of a wider government role in health care.

Washington spends more than double what Britain, France, and Germany do per person on health care, but lags behind other countries in life expectancy and infant mortality, according to the Organization for Economic Cooperation and Development (OECD).