By John Whitesides

WASHINGTON (Reuters) - The U.S. House of Representatives opened debate on Saturday on a sweeping reform bill that would spark the biggest healthcare changes in four decades, with a potentially close final vote expected later in the day.

President Barack Obama headed to Capitol Hill to rally support for his top domestic priority, and Democratic leaders said they were confident they would have the 218 votes needed for approval after striking a middle-of-the-night deal with foes of abortion rights.

Obama and top administration officials had called undecided Democrats on Friday to plead for backing, and House leaders held a series of talks with wavering party members to nail down their support.

The abortion deal allowed dozens of party moderates concerned about the bill's abortion provisions to have a floor vote on whether to impose stronger restrictions on using federal funds to pay for abortions.

The vote on the amendment adds a new element of uncertainty after days of fevered negotiations and intense lobbying. If it is adopted, party leaders hope it will win over many abortion rights opponents without driving off supporters.

"We're confident we're going to have the votes later today," House Democratic leader Steny Hoyer said on MSNBC. "There is some disagreement on some issues but there's universal commitment to passing a healthcare reform bill."

The reform bill would lead to the biggest policy changes in the $2.5 trillion U.S. healthcare system since the creation of the Medicare health program for the elderly in 1965.

House Republicans are united in opposition to the measure, which is designed to rein in costs, expand coverage to millions of uninsured and bar insurance practices such as denying coverage to people with pre-existing conditions.

A POLITICAL RISK

The vote represents a risk for Obama and his fellow Democrats who control Congress. A House victory would be a big step forward for the overhaul and would shift the political spotlight to the Senate, which is preparing its own version.

But failure in the House would be a huge blow to Obama, who has staked much of his political capital on the healthcare battle and has said he wants to sign a bill by year's end.

Democrats can afford to lose 40 of their 258 House members in a vote, but the ranks of potential defectors is much larger.

House Republicans will be given a vote on their alternative healthcare plan before debate on the Democratic bill. The much smaller Republican proposal focuses on controlling costs and curbing medical malpractice lawsuits.

The debate opened to an outburst of intense partisan and procedural wrangling, with Republicans repeatedly objecting to Democratic attempts to enter speeches in the congressional record.

The Democrats' House bill would require individuals to have insurance and all but the smallest employers to offer health coverage to workers. It would set up exchanges where people could choose between private plans and a government-run insurance option, and offer subsidies to help low-income Americans buy insurance.

Congressional budget analysts say the bill would extend coverage to 36 million uninsured people living in the United States, about 96 percent of the population, and would reduce the budget deficit by about $100 billion over 10 years.

"I'm feeling more confident we'll have the votes to pass it," House Energy and Commerce Committee Chairman Henry Waxman said of the bill late on Friday. "Members who looked like they weren't going to be for the bill are coming around."

About 40 moderate House Democrats have said they will oppose the bill unless it ensures federal subsidies are not used to pay for abortions. About 190 members who favor abortion rights said they do not want the bill to exceed current restrictions on using federal money to finance abortions.

Federal law prohibits the use of federal funds to pay for abortions except in the case of rape, incest or to protect the life of the mother.

The amendment offered by abortion rights opponents would ban insurance policies offered on the new exchanges, including the public option, from covering abortions. Those seeking abortion coverage would purchase separate riders with private funds.

(Editing by Arshad Mohammed and Vicki Allen)