Nov. 21 (Bloomberg) -- Senate Majority Leader Harry Reid tonight faces the first big test of whether he can keep his Democratic colleagues united behind health-care legislation.

Senators plan to take a vote at 8 p.m. Washington time that would clear the way for debate on the most sweeping changes to the U.S. health system since the 1965 creation of the Medicare program for the elderly and disabled. Louisiana Senator Mary Landrieu, who had held out on supporting the motion, said today she will vote in favor of debating the bill.

With every Senate Republican opposing the legislation, Reid can’t afford a single defection from his 60-member caucus to enable the chamber to take up the bill when Congress returns from a weeklong Thanksgiving recess. By late yesterday, Democrats had locked up almost all the votes they needed.

“We’re not assuming a thing,” Illinois Senator Dick Durbin, the No. 2 Senate Democrat, told reporters. “We’re working hard to bring all Democrats together.”

Under pressure to finish work on a bill this year, Reid scheduled the rare Saturday vote to begin debate on the $848 billion legislation, which is intended to cover 31 million uninsured people and curb medical costs. Like a bill passed on Nov. 7 by the U.S. House, the Senate measure would require that all Americans get health coverage, setting up new online insurance-purchasing exchanges and providing subsidies.

Winning tonight’s vote would kick in motion Senate debate over some fundamental provisions of the 2,074-page bill, ranging from how strictly to prohibit the use of federal money to fund abortions to how to pay for the legislation.

Last Holdouts

Reid, of Nevada, and Durbin have already persuaded two of three holdouts in their party to support the motion to start debate. Nebraska Senator Ben Nelson intends to vote with Reid, saying it presented “an opportunity to make improvements” to the bill. Nelson said he remains undecided on the measure.

“My vote to move forward on this important debate should in no way be construed by the supporters of this current framework as an indication of how I might vote as this debate comes to an end,” Landrieu said on the floor of the Senate. “I have decided that there are enough significant reforms and safeguards in this bill to move forward, but much more work needs to be done.”

Extra $100 Million

Landrieu may have become more amenable to the legislation after winning the inclusion of an extra $100 million in federal aid for low-income people in her state. She said she still has concerns about the legislation, including that it doesn’t do enough to help small businesses.

Nelson’s and Landrieu’s decisions left Arkansas Senator Blanche Lincoln as a potential “no” vote among Democrats in tonight’s vote. One independent who caucuses with the party, Connecticut Senator Joe Lieberman, has said he supports letting debate begin, although he doesn’t support the current bill.

“I’m still neutral,” she said, telling reporters she wants to have as much “leverage” as possible in the debate. She said that after “leaning against” allowing debate on the bill, she moved “to neutral after meeting with Harry Reid. And that’s where I am right now.”

Lincoln’s Support Likely

Lincoln has signaled she’s likely to go along with Reid, assuring supporters in a Nov. 19 e-mail that even if debate starts, “there will be many days and weeks of efforts to improve” the bill.

The White House urged lawmakers in a statement last night to back the bill, saying it meets the criteria President Barack Obama set for an overhaul.

The legislation “includes critical reforms to the insurance industry, so that Americans will no longer have to worry that they will be denied coverage, or that their coverage will be dropped or watered down when they need it most,” the statement said.

The White House also released a statement from Tommy Thompson, a Republican and onetime Health and Human Services secretary, and former House Democratic Leader Richard Gephardt urging action.

“Americans will look back with appreciation for those who set aside political interests to keep the process moving forward,” Gephardt and Thompson said.

Abortion Stumbling Block

In a sign that abortion will remain a stumbling block to a bill’s passage, the U.S. Conference of Catholic Bishops yesterday sent a letter calling the Senate measure an “enormous disappointment” on the issue. The bishops said the bill did “not live up to Obama’s pledge to bar the use of federal dollars for abortion.”

Senate Democratic leaders have said the measure maintains existing restrictions on the use of taxpayer dollars to fund abortion services.

If the Senate passes a bill, it would work on compromise legislation with the House for a new round of votes in both chambers before a measure could go to Obama.

While the Senate vote tonight is simply on whether to head off Republican tactics to block consideration of the bill, it’s rare for a broader measure to fail after it clears such a hurdle.

The Congressional Research Service released an analysis that found only one instance between 1999 and 2008 when Senate legislation failed after the chamber voted to end a so-called filibuster on a motion to debate a bill. That was a measure designed to protect gun manufacturers from lawsuits that was rejected in 2004.

“When the Senate invoked cloture on a measure itself, it almost always proceeded to a final vote on the measure and passed it,” CRS analysts wrote to Senator Tom Coburn, an Oklahoma Republican who requested the study.

Republican Argument

Republicans said a vote to begin debate is a vote for the legislation.

“No senator who votes for cloture on the motion to proceed tomorrow, I think, can with a straight face contend that they have not somehow embraced the bad policy contained in this bill,” Texas Senator John Cornyn said yesterday.

Still, Lieberman has said he wouldn’t vote for the final legislation unless it’s changed. Lieberman, along with Landrieu, Lincoln and Nelson, have criticized a government-run insurance program to compete with insurers like Hartford, Connecticut- based Aetna Inc.

The only Republican who has supported a health bill so far, Senator Olympia Snowe of Maine, said she won’t side with Reid in the vote on strating debate because of the public option. “I appreciate his phone calls, I’m just saying it hasn’t translated,” she said on Nov. 19.

To contact the reporters on this story: Laura Litvan in Washington at llitvan@bloomberg.net; Kristin Jensen in Washington at kjensen@bloomberg.net

Last Updated: November 21, 2009 13:12 EST