Washington  - Lawmakers are gearing up for the biggest vote yet in the United States on a landmark health reform bill that is designed to cut skyrocketing health costs and extend coverage to millions of Americans who lack insurance.

The House of Representatives is expected to vote Saturday on the health care overhaul, which has been the subject of an extremely divisive political debate for the past year. Prospects for the bill are less certain in the Senate.

President Barack Obama, who has made passing health reform his top domestic priority, on Thursday touted two major endorsements that could boost chances of passing the overhaul by the end of the year.

The American Medical Association (AMA), an advocacy group for doctors, and AARP, which represents seniors, both issued statements in support of the legislation currently before the House.

The two influential groups had been withholding their support amid a divisive debate that has taken up much of Obama's first year in office. Obama said he was "extraordinarily pleased" with the latest endorsements.

Democrats, who control both houses of Congress, are pushing for reforms aimed at bending the cost curve of the world's most expensive health care system, which makes up about 16 per cent of the economy.

The bill is projected to cost 894 billion dollars over 10 years, but claims to reduce the federal budget deficit by 104 billion dollars. It will be paid for through efficiency measures and new taxes on the most expensive insurance plans, according to the non- partisan Congressional Budget Office (CBO).

The bill would also force Americans to buy health insurance and increase government subsidies for the poor. The CBO has said the reforms will increase coverage to about 96 per cent of the population, up from 83 per cent.

"We are closer to passing this reform than ever before," Obama said Thursday, in a surprise appearance at the White House's daily press briefing.

Republicans remain strongly opposed, arguing that the proposals amount to a government takeover of the largely private system. Thousands of opponents protested outside the US Capitol on Thursday.

"This bill is the greatest threat to freedom that I have seen in the 19 years I've been here in Washington," John Boehner, the top Republican in the House, told the protestors.

The House vote is expected to be strictly along party lines. The bill includes a government-run insurance option, which is favoured by left-leaning Democrats but has been a lightning rod for criticism of health reform by conservatives and some moderate Democrats.

The Senate has yet to to begin debating its own version amid a split within the Democratic Party. Some centrists in the party have vowed to block the bill if it includes the government-run option. With Republicans united in opposition, Democrats need every last one of their 60 senators to support the measure.

If passed, the two bills would then have to be reconciled before they can reach the desk of Obama, who has set a deadline of the end of this year for the final legislation to be approved. Senate Majority Leader Harry Reid this week suggested a final bill could slip into 2010.

Politicians have failed for decades to agree on a comprehensive overhaul of the health care system. The last major reform was approved in the 1960s, when Congress created two government-run insurance options: Medicare for seniors and Medicaid for the poor.

The AMA and AARP's backing could inject fresh momentum into the long-running battle in Congress and possibly temper some of the criticism that has been directed at the controversial bill.

AMA President James Rohack said the House version "is not the perfect bill, and we will continue to advocate changes, but it goes a long way toward expanding access to high-quality affordable health coverage for all Americans, and it would make the system better for patients and physicians."

Barry Rand, chief executive of AARP, said the bill met the group's "twin goals of making coverage affordable to our younger members and protecting Medicare." (dpa)