WASHINGTON - The fate of President Barack Obama's top domestic priority -- a remake of the U.S. health-care system -- now rests in the hands of a pivotal but deeply divided Senate committee.

Members of the Senate Finance Committee plan to start voting Tuesday on their version of a health care reform bill.

Democrats on the committee are disappointed with the bill proposed by the chairman, Sen. Max Baucus of Montana. Republicans see a chance to deliver a stunning blow to Obama that could cripple his presidency.

The stakes are so high because this isn't just another committee.

The 23-member committee is a microcosm of the Senate, the narrow gate through which legislation to cover the nearly 50 million uninsured Americans and try to control medical costs has to pass. If the committee can't produce, then the ability of Obama and the Democrats to pass a bill this year will be in serious question.

"If it can't get through the Finance Committee, the mountain that has to be climbed is a much higher mountain, and I don't know whether they'll have the ability to climb that mountain," said Christine Ferguson, a Senate Republican health aide in the 1990s. Now a professor, Ferguson was part of an unsuccessful effort to find a bipartisan deal that might have salvaged President Bill Clinton's health care overhaul.

Baucus, an optimist by nature, says he has the votes. "Oh, yeah -- no doubt," he says.

But last week the chairman stood alone as he explained and defended his 10-year, $856-billion plan.

No Democrats joined him in front of the media -- not even Sens. Kent Conrad of North Dakota and Jeff Bingaman of New Mexico, who spent months working with Baucus trying to find a compromise both political parties could support.

The second-ranking committee Democrat, Sen. Jay Rockefeller of West Virginia, promptly announced he couldn't vote for the bill without major changes.

The Baucus plan would require all Americans to carry health insurance or pay a stiff fine. It would provide subsidies to many middle-class households and expand government health programs for the poor. Insurers could not deny coverage based on someone's personal health history.

The plan would be paid for with cuts in spending on Medicare and Medicaid -- the government-run programs that provide health care coverage to the elderly and poor -- as well as a heavy tax on high-cost health insurance plans. Baucus would not create a government plan to compete with private insurers. And workers at larger companies that offer coverage wouldn't see big changes.

While business and health industry groups generally have said good things about the proposal, core Democratic constituencies are angry.

Labour unions see the insurance tax as a direct threat to hard-won benefits. Liberals are outraged by the absence of a government insurance plan. There's widespread concern that Baucus' subsidies are too meagre and will stick hard-pressed households with thousands of dollars in new insurance bills.

"At the end of the day this has to work for families," said Democratic Sen. Debbie Stabenow of Michigan, a committee member. "The trade-off can't be that a middle-class family won't be able to afford the insurance in this bill."

Baucus can't ignore such concerns. With 13 Democrats and 10 Republicans on the committee, he doesn't have much room to manoeuvr for votes. At best, he may be able to win over one Republican, Sen. Olympia Snowe of Maine.

The committee staff tentatively has scheduled three days of work on the bill, but that may not be enough to handle an expected deluge of amendments -- at least 543.

Democrats say their amendments will be geared to improving subsidies to make coverage more affordable and scaling back or replacing the 35 per cent tax on high-cost health insurance plans.

Baucus would apply the tax to coverage that costs $21,000 a year or more for a family plan, and $8,000 for individual coverage. Sen. John Kerry, a Massachusetts Democrat, said he would like to see the thresholds raised to at least $24,000 and $9,000, respectively. Other senators would like to substitute income tax increases on the wealthy, as Democrats in the House of Representatives have proposed.

Democrats may offer an amendment to add a government-sponsored insurance plan to the bill. But the idea doesn't have much chance because centrists and conservatives outnumber liberals on the committee.

Republicans say they're co-ordinating their amendments to highlight what Sen. John Ensign of Nevada calls "fundamental differences" with Democrats. Some Republican amendments will deal with pocketbook issues.

Sen. Chuck Grassley of Iowa, the committee's top Republican, said Baucus' combination of an insurance requirement and fines as high as $3,800 for going without coverage amounts to "a penalty against middle-class Americans."

Ensign said he wants to limit the future reach of the insurance tax. As proposed by Baucus, the tax would be adjusted based on the general rate of inflation. Medical inflation, however, has been rising about twice as fast. Ensign said that meant the Baucus approach would place more and more insurance plans in the grip of the tax each year.

Republican amendments to bar funds for abortions and tighten rules to prevent benefits from going to illegal immigrants are expected. Baucus says his bill already does both. Republicans are also going to push for a bolder approach on limiting medical malpractice lawsuits.

Many committee members would like the bill that emerges to be significantly different from the plan Baucus placed before them.

But they're up against a hard barrier on costs. Obama has said he wants legislation that costs about $900 billion over 10 years. The Baucus plan is right under that level. Sweeten the subsidies too much and the cost could zoom above $1 trillion.

That's why Democratic leaders and major interest groups backing a health care overhaul are urging the committee to pass a bill now -- and try to work out problems later.

Action by the committee is the first of four big steps before any legislation can be signed into law.

Next, Senate Majority Leader Harry Reid of Nevada would "meld" the Finance Committee bill with a more liberal measure from the Senate Health, Education, Labor and Pensions Committee. Then, the House and Senate would each pass its own version of legislation. Finally, a negotiating committee with representatives from each chamber would have to reconcile the two bills.

"The important thing is to keep moving the process forward, and to keep the big goals in mind, even if there are concerns about the specifics," said John Rother, the top policy strategist for AARP, a lobbying group for older Americans.