WASHINGTON - Democrats pushed ahead toward a make-or-break vote in the House on a massive overhaul of the U.S. health care system on Saturday, boosted by a last-minute appeal from President Barack Obama to pass his top domestic priority.

"This is our moment to deliver," Obama said.

House passage of the bill is crucial if Obama is to fulfil the biggest promise of his campaign last year. But the legislation still faces multiple hurdles and a Senate vote on it might not occur until next year.

Emerging from a closed-door meeting with the president, House Speaker Nancy Pelosi predicted approval of the bill later in the day, adding, "We will pass health care reform."

Meeting with Democrats in a rare Saturday visit to Capitol Hill, Obama said they were approaching what may be their finest moment in politics.

Later, in public remarks in the White House Rose Garden, Obama said, "What's in our grasp right now is a chance to prevent a future where every day, 14,000 Americans continue to lose their health insurance, and every year, 18,000 Americans die because they don't have it."

The bill survived a test vote with a 50-vote margin in a 242-192 roll call, but the vote on final passage was expected to be much closer. Fifteen Democrats joined all 177 Republicans in voting to block the debate.

The bill would cost $1.2 trillion over the next decade. It would provide health coverage to tens of millions of Americans who do not 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 status quo is unaffordable and unsustainable. Health care reform benefits all of us," said Rep. Allyson Schwartz, a Pennsylvania Democrat, as debate opened on the House floor.

Republicans were united in their opposition to the bill which would impact one-sixth of the U.S. economy.

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

The most contentious issue 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.

Obama made his trip to the Capitol complex as abortion rights lawmakers voiced anger at a last-minute concession granted by House Democratic leaders to foes of the procedure, who were given a vote on their proposal for stronger restrictions on abortion coverage.

"There is a risk" that some in the Pro-Choice Caucus would vote against the legislation if the stricter curbs are adopted, said Rep. Diana DeGette, a Colorado Democrat.

The abortion agreement was reached at midnight Friday after hours of intense negotiations brokered by House Speaker Nancy Pelosi.

Anti-abortion Democratic lawmakers fought for and won an opportunity to insert tougher restrictions into the legislation during Saturday's debate, despite fervent opposition from pro-choice liberals who are a driving force behind the overall bill.

"We wish to maintain current law, which says no public funding for abortion," Democratic Rep. Bart Stupak of Michigan said. "We are not writing a new federal abortion policy."

Federal law currently prohibits the use of federal funds to pay for abortions except in cases of rape, incest or situations in which the life of the mother is in danger. Left unresolved is whether individuals would be permitted to use their own funds to buy insurance coverage for the procedure in the federally backed insurance exchange envisioned under the legislation.

Stupak's amendment would deny abortion coverage to anyone who gets federal insurance subsidies or buys a policy from the government, except in cases of rape or incest or if the mother's life is in danger. People could buy separate policies covering just abortions using their own money.

DeGette called Stupak's amendment "the biggest restriction on a women's right to choose that's been considered on the floor of the House" in her 13 years in office.

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.

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

Democrats hold 258 seats in the House and can afford 40 defections and still wind up with 218, a majority if all lawmakers vote.

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 mid-term elections in part because of their failure to enact health care reform legislation.

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.

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 United States is the only developed nation that does not have a comprehensive national health care plan for all its citizens. The 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, 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.