By John Whitesides

WASHINGTON (Reuters) - With a close vote looming, the U.S. House of Representatives was expected 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 on Saturday morning to rally support for the measure ahead of a final vote that Democratic leaders said would be close.

Obama and top administration officials called undecided Democrats on Friday to plead for backing, and House leaders held talks with wavering party members to nail down the 218 votes needed for approval.

Democrats wooed dozens of party moderates concerned about the bill's abortion and immigration provisions, along with its $1 trillion price tag and its possible impact on budget deficits.

"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. "Members who looked like they weren't going to be for the bill are coming around."

The abortion dispute threatened to delay the debate, and Democrats on both sides of the issue negotiated late into the night on Friday in House Speaker Nancy Pelosi's office without reaching a final deal.

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

"The pro-choice caucus will not agree to any language, however you do it, that further restricts a woman's right to choose," said Representative Diana DeGette. The caucus has 190 House members.

The reform bill, Obama's top domestic priority, would lead to the biggest policy changes affecting the $2.5 trillion U.S. healthcare sector 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 restructuring, shifting the political spotlight to the Senate, which is preparing its own version.

Failure in the House would be a blow to the U.S. leader, 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 even larger.

House Democrats plan to open debate on Saturday morning, although there may be a delay if the abortion dispute remains unresolved. A final vote is planned on Saturday night.

But Democratic leaders warned the final vote could be delayed and said the House would keep working through Sunday or later if needed.

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 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.

In an effort to resolve the abortion impasse, Democratic leaders said they would incorporate an amendment by Representative Brad Ellsworth, an opponent of abortion rights, into the rule setting the terms of the debate.

That amendment would guarantee access to insurance plans that agree to refrain from covering abortion. It has been attacked as unsatisfactory by other anti-abortion Democrats and groups like the National Right to Life Committee.

(Editing by Paul Simao)