President Barack Obama's Democratic allies in the Senate strove to lock down support to prevail in remaking the US health care system. Adding to their confidence, a wavering Democrat, Senator Ben Nelson of Nebraska, pictured here in October, said he would vote with his party this time but warned he might side with Republicans in subsequent fights.(AFP/Getty Images/File/Chip Somodevilla)

Senators will vote at 8 p.m. (0100 GMT on Sunday) on whether to start debate on Democratic leader Harry Reid's plan to overhaul the $2.5 trillion healthcare system -- the first procedural hurdle for the 2,074-page bill.

Democrats need 60 votes to approve the motion in the 100-member Senate and have no margin for error -- they control exactly 60 votes and Republicans are united in opposition.

Blanche Lincoln and Mary Landrieu, moderate Democrats from conservative Southern states where the overhaul is unpopular, remain publicly uncommitted. A third wavering Democrat, Ben Nelson, announced on Friday he would support moving to debate.

The healthcare overhaul, President Barack Obama's top domestic priority, would expand coverage to millions of uninsured and bar insurance practices like denying coverage to those with pre-existing conditions.

"This bill provides the necessary health reforms that the administration seeks -- affordable, quality care within reach for the tens of millions of Americans who do not have it today, and stability and security for the hundreds of millions who do," the White House said in a statement of support on Friday.

Republicans have condemned the measure as a costly and heavy-handed government intrusion in the private sector that would raise insurance premiums and increase taxes.

DELAY AND BLOCK

They vowed to delay and block the Senate bill and openly appealed to Lincoln and Landrieu to join them.

"It would be our hope that our more moderate Democratic colleagues would respect the wishes of their constituents rather than do the bidding of Harry Reid," Republican Senator Jon Kyl told reporters.

If the Senate takes up the bill, the debate is expected to begin on November 30 and last at least three weeks, making it unlikely Obama can sign a final bill by the end of the year.

Any differences between a Senate bill and the version passed earlier this month by the House of Representatives would have to be reconciled before a final bill can be voted on again in both houses and sent to Obama.

The legislation would spark the biggest changes in the healthcare system -- which accounts for one-sixth of the U.S. economy -- since the 1965 creation of the Medicare government health insurance plan for the elderly.

The Senate bill would require all Americans to buy insurance and would set up exchanges where they could choose among various options. It would offer subsidies to help low-income workers pay for the coverage.

Republicans have criticized its tax increases to help pay for the expanded insurance coverage. It would raise the Medicare payroll tax on high-income workers, which is used to finance Medicare, and impose a tax on high-cost "Cadillac" insurance plans.

"Higher premiums, higher taxes on the middle class, including small businesses, Medicare cuts -- there's nothing here to like," Republican Senator John Cornyn said. "What we need to do is kill this bill."

(Editing by Peter Cooney)