Democratic leaders were hopeful they could pass the bill late Saturday, giving Obama his biggest victory since last November's historic election. Obama made a rare visit to Congress earlier in the day to meet with House Democrats in an effort to cement a majority to pass the legislation that is his top domestic priority.

Even if the House passes a health care bill, it by no means guarantees that any reform measure will reach the president's desk to sign. It is still has to pass in the Senate.

The bill would provide health coverage to tens of millions of Americans who don't have it now, require most employers to offer it to their workers and prohibit insurance companies from denying coverage based on a person's medical history.

The United States is the only developed nation that does not have a comprehensive national health care plan.

"The status quo is unaffordable and unsustainable. Health care reform benefits all of us," said Democratic Rep. Allyson Schwartz, as debate opened on the House floor.

Republicans were united in their opposition to the bill, using procedural delays to slow the debate.

"The American people need to understand this is about a government takeover of the whole health care system," said Rep. Paul Broun.

The most contentious issue in the 10-year, US$1.2 trillion House bill is a new government-run insurance plan that would be offered alongside private coverage within new purchasing marketplaces, or "exchanges," where individuals and small businesses could shop for and compare options.

In the runup to a final vote, conservatives from the two political parties joined forces to impose tough new restrictions on abortion coverage in insurance policies to be sold to many individuals and small groups. they prevailed on a roll call of 240-194.

Ironically, that only solidified support for the legislation, clearing the way for conservative Democrats to vote for it.

However, abortion-rights supporters were outraged at the concession to abortion opponents. Rep. Diana DeGette, a Democrat, said "there is a risk" that some in the Pro-Choice Caucus would vote against the bill as a result.

The leadership's hope is that no matter how the vote on the abortion measure turns out, Democrats on both sides of the abortion divide will then unite to give the health care bill a majority over unanimous Republican opposition.

The House, meanwhile, rejected a Republican health overhaul bill on Saturday by a vote of 258-176. The Republican version that would have lowered costs for people with insurance but done little or nothing to expand coverage to those without any.

With Democrats' command of the necessary votes looking tenuous, Obama threw the weight of his administration behind the effort to round up support. He and top administration officials worked the phones to pressure wavering lawmakers.

Democratic Rep. Jason Altmire said he heard Friday from Obama, White House Chief of Staff Rahm Emanuel, Health and Human Services Secretary Kathleen Sebelius and Education Secretary Arne Duncan.

Their message: "This is a historic moment. You don't want to end up with nothing," said Altmire, who remained undecided.

Democrats hold 258 seats in the House and can afford 40 defections and still wind up with 218, a majority if all lawmakers vote. But all 177 Republicans were expected to vote "no," and Democratic leaders faced a series of complications trying to seal the needed votes for their complex and controversial legislation that would affect one-sixth of the economy and touch the lives of countless Americans.

Democrats have little room for error, with the prospect of the 2010 congressional elections looming large and a some of their own moderates already declaring their opposition.

Obama's push to reform the health care system has already moved further along the legislative process than a similar effort by President Bill Clinton in his first-term in the 1990s. Democrats are mindful that the party lost control of Congress in the 1994 elections in part because of their failure to enact health care reform legislation.

House rules make it easier for the large Democratic majority to advance legislation. But it will be tougher to get Senate approval because Democrats will need 60 out of 100 votes to end debate and bring legislation to a final vote, and several moderate Democratic senators still have reservations. If the Senate does pass a bill, it would have to be reconciled with the House version by a panel of lawmakers from both chambers before the legislation is put up for final approval.

The U.S. government provides coverage for the poor, elderly and military veterans, but most Americans rely on private insurance, usually provided through their employers.

But with unemployment climbing above 10 percent, many Americans are losing their health insurance when they lose their jobs. At the same time, the deepening budget deficit has made it difficult for lawmakers to support costly new programs.