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.