WASHINGTON -- After negotiating critical last-minute commitments, Senate Democratic leaders yesterday stood on the verge of achieving the necessary 60 votes to begin consideration of the most expansive health care legislation to go before the Senate in nearly half a century.

Nebraska Sen. Ben Nelson, who was among three Democratic holdouts, announced that he would back an all-important procedural vote set for today that will allow the chamber to take up the wide-ranging bill unveiled this week by Senate Majority Leader Harry Reid, D-Nev.

Democratic leaders expect Sens. Blanche Lincoln of Arkansas and Mary Landrieu of Louisiana to support a cloture vote on the "motion to proceed," although the two lawmakers have not formally announced their plans.

With the backing of those three senators, Democratic leaders are all but assured of clearing the procedural hurdle, a key step if Congress is to send President Barack Obama a health care bill by the end of January, as party leaders hope.

The Senate would start formal debate on Democrats' top domestic priority when lawmakers come back Nov. 30 from their Thanksgiving recess -- beginning with consideration of a series of proposed amendments from both sides of the aisle.

The House has already passed its version of health care legislation, which will ultimately have to be reconciled with the Senate bill.

The Senate measure, which would cost $848 billion over 10 years, is designed to eventually expand coverage to another 31 million Americans, while restraining federal deficits and taking steps to make the nation's health care system more efficient and reliable for patients.

It is funded by a politically delicate mix of cuts to the federal Medicare system and new taxes on health care industries, high-end "Cadillac" health plans and wealthy households.

The measure is so politically charged that finding the votes even to take up debate on the legislation turned into a Capitol drama that dragged on for weeks. With Republican lawmakers determined to filibuster every stage of the legislative process, all 58 Democratic senators and the two independents who caucus with them must hold together to move any health care legislation.

That has forced the majority leader, Mr. Reid, a veteran parliamentary strategist, to cut numerous side deals to satisfy demands of individual lawmakers in his caucus. Mr. Reid included language in his bill that would boost aid for Louisiana's Medicaid insurance program for poor people in a bid for Ms. Landrieu's support. He also slashed proposed new taxes on the medical-device industry to ease the concerns of Democrats from states that are home to large device makers, such as Indiana's Sen. Evan Bayh.

And yesterday, Mr. Reid struck a deal with Oregon Sen. Ron Wyden, who has been pressing to allow more Americans access to new insurance exchanges, where commercial insurers would compete to offer plans to consumers who do not get health benefits through work. Mr. Wyden's proposal, which Mr. Reid agreed to add to the bill, would open the exchanges to about a million workers who could not afford the health plans offered by their employers, according to an estimate by the nonpartisan Congressional Budget Office.

Republican lawmakers have kept up a steady effort to make it more difficult for conservative Democrats to vote to open debate, casting the parliamentary move as a referendum on the health care bill itself. This "vote is something we need to look at as a vote that's not some sort of ... a procedural vote," Sen. Judd Gregg, R-N.H., said on the Senate floor yesterday. "It's a substantive vote on whether or not we're going to fundamentally change the way health care is delivered in this country."

Mr. Nelson firmly rejected that characterization. "It is only to begin debate and an opportunity to make improvements," he said in a statement yesterday. "If you don't like a bill, why block your own opportunity to amend it? ... I won't slam the doors of the Senate in the face of Nebraskans now. They want the health care system fixed. The Senate owes them a full and open debate to try to do so."

Mr. Nelson has indicated unease about several key components of the legislation, including creation of a new government insurance plan and restrictions on federal funding for abortion, which he said he wants to see strengthened.

Demands like these, which figure to define the upcoming debate, will likely complicate Mr. Reid's attempt to pass the health care legislation, even if he prevails on the procedural motion. Senate rules will require Mr. Reid to cobble together 60 votes again to end debate on the health care bill and bring it up for a final vote, which he hopes to do before Christmas.

Several lawmakers who oppose the government insurance plan, including Sen. Joe Lieberman, I-Conn., who caucuses with Democrats, said they would not vote for a bill unless the "public option" provision is removed. They may back an amendment during the upcoming debate that would create a "trigger." Under such an arrangement, a government plan could only be created in parts of the country where commercial insurers did not meet benchmarks for quality and affordability.

Liberal lawmakers plan to seek other changes, including more aid to low-income Americans to help them buy health insurance. Sen. Bernie Sanders, I-Vt., who also caucuses with Democrats, said that while he will support today's procedural vote, his support for a final bill is "not at all guaranteed."

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.