As the Senate convened in a blizzard, Democratic leaders hailed a breakthrough that came when Senator Ben Nelson, Democrat of Nebraska, agreed to back the bill after 13 hours of negotiations on Friday, making him the pivotal 60th vote for a measure that President Obama has called his top domestic priority.
"Change is never easy, but change is what's necessary in America," Mr. Nelson said at a morning news conference. "And that's why I intend to vote," he said, "for health care reform."
Mr. Obama, appearing on television from the White House, said: "Today is a major step forward for the American people. After nearly a century-long struggle, we are on the cusp of making health care reform a reality in the United States of America."
The blinding snow outside the Capitol added to what had already been a chaotic few weeks for the Senate, which has met every day since Nov. 30 and prepared to work through its third consecutive weekend. The sergeant-at-arms had four-wheel drive vehicles at the ready to bring lawmakers in for votes. And while senators wore the jackets and ties required on the Senate floor, dress shoes gave way to boots.
Mr. Nelson committed his vote after winning tighter restrictions on insurance coverage for abortions, as well as increased federal health care aid for his state.
With Senate leaders increasingly confident that they would pass the bill, Mr. Nelson pointedly warned that he would oppose the final version if negotiations with the House, which approved its bill last month, result in changes that he does not like.
But House liberals are expected to resist some concessions made in the Senate, including the dropping of a proposed government-run health insurance plan, or public option, to secure the votes of centrist hold-outs.
The legislation, the most significant overhaul of the nation's health care system in more than a generation, seeks to extend health benefits to more than 30 million uninsured Americans.
Because the Democrats nominally control 60 seats in the Senate -- the precise number needed to overcome Republican filibusters -- every senator in the Democratic caucus effectively has veto power over the bill. No Republican is willing to support it.
"The lines are drawn," said Senator Richard M. Burr, Republican of North Carolina. "He has to get 60 votes. If he doesn't get 60 votes, the American people win. If he does get them, America's payback will come in the form of the 2010 elections."
Not all Democrats have publicly said they will vote for the bill, but Senate leaders and senior White House officials believe they have agreement.
"All Senate Democrats stand shoulder to shoulder with President Obama and the American people, who know that inaction is not an option," the majority leader, Senator Harry Reid of Nevada, said at a news conference.
Faced with Republican opposition that many Democrats saw as driven more by politics than policy disagreements, Senate Democrats in recent days gained new determination to bridge differences among themselves and prevail over the opposition.
Lawmakers who attended a private meeting between Mr. Obama and Senate Democrats at the White House on Tuesday pointed to remarks there by Senator Evan Bayh, Democrat of Indiana, as providing some new inspiration.