President Obama’s rare appearance on the Hill was part of an all-out Democratic effort to rally Democrats to support the biggest health care legislation since the creation of Medicare for the elderly in 1965.

The debate opened after months of internal party wrangling, angry town-hall-style meetings and committee deliberations.

Democrats said their proposal would vastly improve the ability of Americans to gain affordable health insurance.

“Now is the chance to fix our health care system and improve the lives of millions of Americans,” said Representative Louise M. Slaughter,Democrat of New York and chairwoman of the Rules Committee, as she opened the daylong debate.

Republicans labeled the measure a government takeover of health care and said it would cost jobs in the struggling economy.

“This bill would mortgage the future of my grandkids,” said Representative Roscoe G. Bartlett, Republican of Maryland.

The expected wall of Republican opposition gave Democrats little room to maneuver and they were working to corral as many party members as they could.

Democrats remained optimistic they could pass the legislation, which would extend coverage to 36 million Americans without insurance and seeks to end what they see as abusive insurance industry practices, including the denial of coverage because of a pre-existing conditions.

But the difficult issue of how much to restrict new federal spending on abortion complicated the outcome by creating a split between Democrats supporters and opponents of abortion rights.

Unable to reach agreement on compromise language on abortion coverage, House leaders decided early Saturday to allow a vote on tight restrictions that would prohibit federal money from being used to pay for abortions, either through a new federal health insurance plan or under private plans that enroll people relying on federal subsidies.

“From Day 1, my goal has been to ensure federal tax dollars are not used to pay for abortions,” said Representative Brad Ellsworth, Democrat of Indiana and one of the authors of the abortion provision.

The abortion restrictions were bitterly opposed by Democrats who support abortion rights and may face a choice of either accepting the restrictions or scuttling the entire health measure that Democrats have spent months negotiating and assembling. Several Democrats said they expected those lawmakers to go along grudgingly with the anti-abortion proposal in the hope of changing it during later negotiations with the Senate.

“There’s no way at the end of the day we’re going to support these kinds of further restrictions on abortion,” Representative Jan Schakowsky, Democrat of Illinois, said in an interview on C-Span. Schakowsky said that she and others would try to defeat the abortion proposal. But if they are unsuccessful, she said, “we want to move the process along.”

The looming House vote was a significant step in the long-sought Democratic goal of enacting broad changes in the way health care is delivered in the nation, but it was only a first step. The Senate has yet to bring its own emerging measure to the floor for debate, and should both the House and the Senate advance a measure, they will still need to negotiate and approve a final bill in the weeks ahead.

The struggle House Democrats had in lining up the minimum number of votes for the measure was a clear indication of how difficult it will be to get final legislation to the president’s desk.

The House legislation, running almost 2,000 pages long, would cost $1.1 trillion over 10 years and would require most Americans to obtain health insurance or face penalties. Insurers could not reject people because of pre-existing medical conditions and could not charge higher premiums because of a person’s health status or sex. The bill would trim Medicare payments to hospitals and many other health care providers and would impose a new tax surcharge on high-income people.

Most employers would have to provide coverage or pay a tax penalty up to 8 percent of payroll. The bill would significantly expand Medicaid and would offer subsidies to help moderate-income people buy insurance from private companies or from a new government insurance plan.It would also set up a national insurance exchange where people could shop for coverage.

Republicans planned to force a House vote on a much more modest plan that would expand coverage to just three million of the uninsured. But its authors said it would bring down the costs of private insurance premiums, which they said was the chief concern of most Americans.

“More taxes, more spending and more government is not the plan for reform the people support,” said Representative Virginia Foxx, Republican of North Carolina and one of the conservatives who relentlessly assaulted the Democrats’ plan.

Democrats said their proposal was long overdue, would relieve the mounting anxiety of Americans struggling to get and retain health insurance, and would ultimately improve the national economy by bringing spiraling health care costs under control.

“Our plan is not perfect, but it is a good start toward providing affordable health care to all Americans,” said Representative Peter A. DeFazio, Democrat of Oregon.

Among the final changes Democrats agreed to was using the measure to repeal a federal antitrust exemption for health insurance companies.