WASHINGTON - The long, turbulent struggle over landmark health care legislation tilted unmistakably in President Barack Obama's favour as one by one, undecided Democratic lawmakers began choosing sides Friday.

In full campaign mode, his voice rising, the president all but claimed victory, declaring to a cheering audience in Virginia, "We are going to fix health care in America."

With the showdown vote set for Sunday in the House of Representatives, Obama decided to make one final, personal appeal to rank-and-file Democrats, arranging a Saturday visit to the Capitol.

Obama has put his presidency on the line to gain passage of his top domestic priority in the face of unanimous opposition from Republicans who say the plan amounts to a government takeover of health care that will lead to higher deficits and taxes.

The health care reform program would affect nearly every American and remake one-sixth of the U.S. economy. For the first time Americans would be required to have health care insurance and face penalties if they refused. The United States is the only major industrialized country that does not have a comprehensive national health care plan.

Obama, who delayed a trip to Indonesia and Australia to help ensure passage of the legislation, spoke to an enthusiastic crowd at a suburban university, lobbing attacks at the insurance industry with his jacket off and sleeves rolled up.

"The only question left is this: Are we going to let the special interests win once again, or are we going to make this vote a victory for the American people?" he said.

Under a complex and controversial procedure Democrats have devised, a single vote will likely be held in the House to endorse a bill approved by the Senate last year as well as a second measure with a package of fixes agreed to in negotiations with the White House.

The Senate would then use a procedure called reconciliation to pass the fix-it measure that requires only a simple majority of 51 in the 100-member body, avoiding Republican delaying tactics.

One day after House Democrats released 153 pages of revisions to their bill, Democratic leaders were scrambling to gather the 216 votes they needed for passage, so every undecided lawmaker was the focus of personal attention from the leader of the House, Speaker Nancy Pelosi, and the White House.

They focused lobbying efforts on two separate groups of Democrats, 37 who voted against an earlier health care bill in the House and 40 who voted for it only after first making sure it would include strict abortion limits that now have been modified.

So far at least six Democratic House members have switched from opposition to favouring the measure. They include freshman Rep. John Boccieri of Ohio who said Friday, "I'm not worried about the election. I'm worried about doing what's right." The House Republican campaign committee issued a tart warning, predicting the Ohio Democrat's swift political demise in the November election.

On the other side of the ledger, Rep. Michael Arcuri became the first Democratic former supporter to announce his intention to oppose the bill. Rep. Anh Cao, the only Republican to support the earlier measure, has also announced his opposition.

Several abortion opponents announced they would vote in favour of the new bill, despite the changes it included, and there was talk among others of finding a largely symbolic way that would allow them to follow.

The political ramifications remained to be fought out in the November elections when control of Congress will be at stake. Republican leaders vowed to make Democrats in swing districts pay for supporting the legislation.

Republicans and their allies unleashed a fresh barrage of criticism, warning the bill would eviscerate a private Medicare health program that serves 10 million elderly and impose new burdens on business in time of recession. But they stopped well short of predicting they could stop the bill, and there were questions about the authenticity of a purported Democratic strategy memo they circulated in an effort to raise doubts about the legislation.

The legislation, more than a year in the making, would extend coverage to an estimated 32 million Americans who lack it, ban insurers from denying coverage on the basis of pre-existing medical conditions, and cut deficits by an estimated $138 billion over a decade. Congressional budget analysts estimated the cost of the two bills combined was $940 billion over a decade.

Billions were set aside for subsidies to help families at incomes of up to $88,000 a year afford the cost of insurance, and the legislation also provides for an expansion of Medicaid that will give government-paid health care to millions of the poor.

The largest association of U.S. doctors and the biggest senior citizens' lobby endorsed Obama's revised health overhaul legislation.

James Rohack, president of the American Medical Association, said that the pending bill is not perfect, but it is the next step toward real reform of the U.S. health care system.

"This is certainly not the bill we would have written, but we cannot let the perfect be the enemy of the good," Rohack said.

The American Association of Retired People, known by its initials as AARP, has steadily supported health overhaul efforts. The organization's chief executive A. Barry Rand sent a letter Friday to members of the House, urging them to vote yes.

Republicans circulated a letter from Caterpillar company vice-president Gregory S. Foley to House leaders, warning that passage of the legislation would raise the earth-moving equipment company's health care costs by "more than 320 per cent (over $100 million) in the first year alone and put at risk the coverage our current employees and retirees receive."