WASHINGTON — After negotiating critical last-minute commitments, Senate Democratic leaders Friday 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 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 from their Thanksgiving recess Nov. 30.

The House of Representatives 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 of the 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, Reid, a veteran parliamentary strategist, to cut numerous side deals to satisfy the demands of individual lawmakers in his caucus. Reid included language in his bill that would boost aid for Louisiana’s Medicaid insurance program for poor people in a bid for 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 device makers, such as Indiana’s Sen. Evan Bayh.

And on Friday, 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.

Republican lawmakers have kept up a steady effort to make it more difficult for conservative Democrats to vote to open debate.

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 Friday. “It’s a substantive vote on whether or not we’re going to fundamentally change the way health care is delivered in this country.”

“It is only to begin debate and an opportunity to make improvements,” Nelson said in a statement Friday.

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

Demands like these, which figure to define the upcoming debate, will likely complicate Reid’s attempt to pass the health care legislation even if he prevails on the procedural motion.

The rules of the Senate will require 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, Ind- Conn., 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.