By John Whitesides

WASHINGTON (Reuters) - With a close vote looming, the U.S. House of Representatives was set to open debate on Saturday on a sweeping reform bill that would spark the biggest healthcare changes in four decades.

President Barack Obama planned to visit Congress to rally support for the measure, his top domestic priority, ahead of a final vote expected later on Saturday.

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 the 218 votes needed for approval.

Democrats struck a deal in middle-of-the-night negotiations allowing dozens of party moderates concerned about the bill's abortion provisions a floor vote on whether to impose stronger restrictions on using federal funds to pay for abortions.

The vote on the amendment added more uncertainty to what already promised to be a close vote, but Democratic leaders hoped party members on both sides of the issue would unite.

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

The reform bill, Obama's top domestic priority, 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 huge 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 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.

House Democrats plan to open debate on Saturday morning, but Democratic leaders warned the House would keep working through Sunday or later if needed to reach a final vote.

Democrats can afford to lose 40 of their 258 House members in a vote, but the ranks of potential defectors is even 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 Democrats' House bill would require individuals to buy insurance and all but the smallest employers to offer health coverage to workers. It would offer subsidies to help low-income Americans buy insurance and eliminate the healthcare industry's exemption from federal antitrust laws.

Congressional budget analysts say the bill would extend coverage to 36 million uninsured people living in the United States 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 will not allow 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. Abortion rights opponents said their amendment would retain current law.

(Editing by Arshad Mohammed and Vicki Allen)