WASHINGTON (Reuters) - The White House on Sunday sought to preserve the fragile alliance of Democratic liberals and moderates backing broad healthcare reform legislation, with tough decisions looming on abortion and a new government-run insurance program.

Republicans vowed to continue to fight the measure, but admitted they probably were helpless to stop it in the Senate.

Senate Democrats planned a series of crucial procedural votes scheduled to begin at 1 a.m. on Monday, with debate possibly concluding with final Senate passage on Christmas Eve on Thursday.

Asked if Republican senators could do anything to stop the Senate from passing it by Christmas Eve, Republican Senator John McCain told “Fox News Sunday,” “Probably not. But what we can do is continue winning the battle of American public opinion.”

The White House predicted that the bill, President Barack Obama’s top legislative priority, will win final congressional passage, and called it a major achievement even if it does not give Obama and his fellow Democrats everything they want.

“While it is not perfect, the bill pending in the Senate today is not just good enough -- it is very good,” Vice President Joe Biden wrote in a New York Times opinion piece.

Democratic holdout Ben Nelson announced his support for the Senate legislation on Saturday after securing language aimed at ensuring federal funds are not used to pay for abortions and winning extra healthcare funds for his home state of Nebraska.

Nelson’s support gave Senate Majority Leader Harry Reid the 60 votes he needs in the 100-seat Senate to pass Obama’s top domestic priority by Christmas.

But Democrats still have a lot of hard work ahead as they look toward ironing out differences between the healthcare bill already passed by the House of Representatives last month and the version Senate Democrats hope to pass this week.

The Senate bill would extend coverage to 30 million uninsured Americans, expand the Medicaid government health insurance program for the poor, provide subsidies to help some people pay for coverage and halt industry practices like refusing insurance to people with pre-existing medical conditions.

House and Senate negotiators will have to work out abortion language that satisfies abortion opponents like Nelson and Representative Bart Stupak without chasing off liberal abortion rights supporters. Stupak, who pushed more restrictive language on abortion in the House bill, said the compromise Senate language crafted by Nelson was “not acceptable.”

PUBLIC OPTION

Democrats must decide on including a government insurance program to compete with private insurers. Liberals want the “public option,” which is in the House bill, but not the Senate one. And they will have agree on how to fund the reform, with the House and Senate versions taking two different approaches.

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Nelson told CNN’s “State of the Union” program on Sunday that if the final bill that emerges from the House-Senate negotiations includes a public option, he could not vote for it. He also indicated that if the legislation is paid for the way the House measure proposes, “That would break it.”

Democratic Senator Kent Conrad, a fiscal hawk and chairman of the Budget Committee, said the final bill will have to hew closely to the Senate’s version in order win final passage.

“Anybody who’s watched this process can see how challenging it has been to get 60 votes,” he said on “Fox News Sunday.”

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Congress has been tied up for months in acrimonious debate over healthcare reform legislation, with Republicans saying the Democratic measure is too costly and too intrusive into the healthcare sector.

Healthcare costs devour 16 percent of the U.S. economy -- burdening states and the federal government while also hurting the competitiveness of U.S. businesses -- even as tens of millions remain with no public or private health insurance.

White House senior advisor David Axelrod predicted congressional passage but declined to say when he thought the two chambers would iron out their significant differences, or which version he preferred.

“I think it will pass the Congress,” Axelrod said on NBC’s “Meet the Press.” “I think we’re going to get it done.”

Republicans said they would continue to erect procedural roadblocks even with Senate passage likely.

More improvements are possible after the bill becomes law, Axelrod said. The Obama administration will seek to allow Americans to buy prescription drugs that have been imported from other countries such as Canada, where medicines often cost less, he said. The Senate turned back efforts to include drug re-importation in the healthcare bill.