WASHINGTON — Jubilant Democrats locked in Nebraska Sen. Ben Nelson as the 60th and decisive vote for historic health care legislation today, putting President Barack Obama’s signature issue firmly on a path for Christmas Eve passage.

At the White House, Obama swiftly welcomed the breakthrough, saying, “After a nearly century-long struggle, we are on the cusp of making health care reform a reality in the United States of America.”

In the Capitol, Senate Majority Leader Harry Reid smiled broadly when asked if Nelson’s decision gave him the 60-vote majority necessary to overcome solid Republican opposition. “Seems that way,” he said. The Nevada Democrat agreed to a series of concessions on abortion and other issues demanded by Nelson in daylong talks on Friday, then informed Obama of the agreement in a late night phone call as the president flew home from climate talks in Copenhagen.

The Congressional Budget Office said the Senate bill would extend coverage to more than 30 million Americans who lack it. It also imposes new regulations to curb abuses of the insurance industry, and the president noted one last-minute addition would impose penalties on companies that “arbitrarily jack up prices” in advance of the legislation taking effect.

CBO analysts also said the legislation would cut federal deficits by $132 billion over 10 years and possibly much more in the subsequent decade.

The developments unfolded on a day of improbables — a snowstorm enveloped the Capitol, creating whiteout conditions outside; while inside senators staged dueling news conferences as if their presence on the Saturday before Christmas was the rule rather than the rarest of exceptions.

At its core, the legislation would create a new insurance exchange where consumers could shop for affordable coverage that complied with new federal guidelines. Most Americans would be required to purchase insurance, with federal subsidies available to help defray the cost for lower and middle income individuals and families.

In a concession to Nelson and other moderates, the bill lacks a government-run insurance option of the type that House Democrats inserted into theirs. In a final defeat for liberals, a proposed Medicare expansion was also jettisoned in the past several days as Reid and the White House maneuvered for 60 votes.

Outnumbered Republicans unleashed a new series of attacks against the legislation and vowed to delay its passage as long as possible. The next — and most critical — test vote was set for about 1 a.m. Monday.

To secure passage, Democrats will need to show 60 votes on two additional occasions, and in the meantime, Reid made sure Republicans would have no additional chances to seek changes to the measure.

“This bill is a legislative train wreck of historic proportions,” the party’s leader, Sen. Mitch McConnell of Kentucky, said at a news conference. He pointed to cuts to Medicare that CBO said totaled more than $470 billion over a decade, with reductions in planned payments to home health care agencies and hospices. He also said the bill includes “massive tax increases” at a time of double-digit unemployment.

Republicans also noted that CBO concluded that under the bill, “federal outlays for health care would increase during the 2010-2019 period, as would the federal budgetary commitment to health care.”

True to their word, Republicans objected when Reid sought permission for Nelson to announce his decision in a speech on the Senate floor, then insisted clerks read aloud 383 pages of last-minute changes the majority leader unveiled.

Many of Reid’s revisions were designed to secure the 60 votes needed to steer the bill past the GOP filibuster.

Those drafted at Nelson’s behest drew the most attention, and included further restrictions on abortion coverage in policies sold inside the exchanges.

States would be permitted to ban insurance coverage of abortions in policies sold in the exchange, except in cases of rape, incest or when the life of the mother is in jeopardy. In states where such coverage is permitted, consumers must notify their insurance company they want it, and pay for it separately.

The Nebraskan also won increased federal funds to cover his state’s cost of covering an expanded Medicaid population at a cost that one Democratic official put at $45 million over a decade, and took credit for easing the bill’s impact as well as other, smaller changes.

When he finally announced his decision, Nelson did so at a news conference in a Capitol corridor, rather than from his Senate desk as Democrats had preferred. Noting the bruising negotiations over abortion, he said, “I know this is hard for some of my colleagues to accept and I appreciate their right to disagree. But I would not have voted for this bill without these provisions.”

Senators who support abortion rights accepted the changes reluctantly, but conservative Republicans and the National Right to Life Committee criticized them as a step backward from the equivalent part of the House bill.

Rep. Bart Stupak, D-Mich., who pushed through the restrictions in the House-passed bill, also rejected Nelson’s deal. He called it “not acceptable” because it “would allow the federal government to subsidize insurance policies with abortion coverage.” He said he intends to keep working to find a solution that would allow him to ultimately vote for the health care bill.

Nelson, Nebraska’s former state insurance commissioner, wasn’t the only squeaky senatorial wheel within the 60-member Democratic caucus.

Sen. Bernie Sanders, I-Vt., claimed credit for a last-minute, $10 billion increase in funding for community health centers nationwide, which he said would create new or expanded facilities in 10,000 areas and provide primary care for 25 million more Americans.

Sanders made an impassioned speech on the Senate floor earlier in the week on behalf of a doomed proposal for government-run health care. In an interview, he said he only agreed to vote for the legislation on Friday, when Reid told him additional billions for the health centers would be included.

Another provision in Reid’s changes provides additional federal funding for hospitals in Montana, North Dakota, Wyoming and the Dakotas, although no cost estimate was available.

The House passed its version of the bill in November, and final compromise talks are expected to begin after a brief break for the holidays. Numerous issues must be resolved — including the role of government in the new insurance market and abortion restrictions. But Democrats have made a point all year of compromising on difficult issues in the name of the most far-reaching changes in the nation’s health care system in generations, and hope to have a bill for Obama to sign before next month’s State of the Union address to Congress.

In place of a government-run insurance option, the estimated 30 million Americans purchasing coverage through new insurance exchanges would have the option of signing up for national plans overseen by the same office that manages health coverage for federal employees and members of Congress. Those plans would be privately owned, but operated on a nonprofit basis, as many Blue Cross Blue Shield plans are now.

Insurance companies would be barred immediately from denying coverage to children because of a pre-existing health condition. The prohibition on denial of coverage for adults would not take effect in the Senate bill until 2014, a disappointment for consumer advocates.

Among the changes Reid incorporated was dropping a proposed tax on cosmetic surgical procedures, including Botox injections. Instead, Senate Democrats are proposing a 10 percent sales tax on tanning salons, to be paid by the person soaking up the rays. The Food and Drug Administration says ultraviolet radiation from tanning can increase the risk of skin cancer.

The revised bill also calls for a .9 percent increase in the Medicare payroll tax on incomes over $200,000 for individuals and $250,000 for couples. Reid’s earlier bill had a smaller hike, .5 percent.

The bill also taxes high-cost insurance plans as part of a plan to put downward pressure on health care use.