Thirteen Republicans broke with their top two leaders and joined 56 Democrats and two independents in providing the necessary two-thirds vote to approve the treaty. The vote was 71-26, with Sen. Ron Wyden, D-Ore., showing up just two days after cancer surgery.
The accord, which still must be approved by Russia, would restart onsite weapons inspections as successors to President Ronald Reagan have embraced his edict of "trust, but verify."
Russian Foreign Minister Sergey Lavrov said Moscow welcomed the vote but still needed to study the accompanying Senate resolution. Vice President Joe Biden presided over the Senate and announced the vote. Secretary of State Hillary Rodham Clinton observed the vote from the Senate floor. Both former senators had lobbied furiously for the treaty's approval.
"The question is whether we move the world a little out of the dark shadow of nuclear nightmare," Foreign Relations Committee Chairman John Kerry said to his colleagues moments before the historic tally.
Kerry, D-Mass., said the vote was appropriate "in a season that celebrates and summons us to the ideal of peace on earth."
Calling the treaty a national security imperative, Obama had pressed for its approval before a new, more Republican Congress assumes power in January. In recent days, he had telephoned a handful of wavering Republicans, eventually locking in their votes.
The Obama administration has argued that the United States must show credibility in its improved relations with its former Cold War foe, and the treaty was critical to any rapprochement. The White House is counting on Russia to help pressure Iran over its nuclear ambitions.
The New START treaty, signed by Obama and Russian President Dmitry Medvedev in April, would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200. It also would establish a system for monitoring and verification. U.S. weapons inspections ended last year with the expiration of a 1991 treaty.
"START" stands for Strategic Arms Reduction Treaty.
Obama overcame the opposition the Senate's top two Republicans - Minority Leader Mitch McConnell of Kentucky and Jon Kyl of Arizona, the GOP point man on the treaty.
Peeved by the Democrats' interruption of the eight days of treaty debate for other legislation, McConnell accused the White House of politicizing the process.
McConnell said national security was the main concern, "not some politician's desire to declare a political victory and hold a press conference before the first of the year."
The ratification was a turnaround for a treaty whose fate was uncertain just a month ago. Conservatives railed that the pact would limit U.S. options on missile defense, lacked sufficient procedures to verify Russia's adherence and deserved more time for consideration than the abbreviated postelection session.
Republican Sen. Mark Kirk of Illinois, who won Obama's Senate seat, dismissed the treaty for imposing "marginal reductions in the Russian arsenal."
The fierce opposition diminished quickly as former Presidents George H.W. Bush and Bill Clinton, six former Republican secretaries of state and much of the nation's military and foreign policy experts called for the treaty's ratification.
Defense Secretary Robert Gates and Joint Chiefs of State Chairman Adm. Mike Mullen pressed for approval, with Mullen simply telling senators earlier this week, "the sooner, the better."
Weeks after Republicans routed Democrats at the polls - seizing control of the House and strengthening their numbers in the Senate - Obama has prevailed in securing overwhelming bipartisan approval of a tax deal with Republicans, getting repeal of the 17-year-old ban on openly gay military members and winning approval of the treaty.
The treaty capped a hefty yearlong record of legislation for the Democratic-controlled Congress and Senate Majority Leader Harry Reid, D-Nev. - a massive overhaul of the health care system, new financial regulations and a food safety bill as well as the postelection measures.
The treaty vote exposed divisions within the Republican Party that could stretch into the 2012 presidential and congressional elections. Obama got the treaty with the help of several GOP Senate moderates who split with possible White House hopefuls, some of the fiercest critics of the accord.
Former Massachusetts Gov. Mitt Romney opposed the pact; Sen. Scott Brown, R-Mass., who faces re-election in 2012, voted for it. Former Alaska Gov. Sarah Palin said the treaty was not in the country's interest; Sen. Lisa Murkowski, R-Alaska, backed it. Former House Speaker Newt Gingrich described it as an "obsolete approach that's a holdover from the Cold War;" Sen. Johnny Isakson, R-Ga., supported it.
In mid-November, the treaty's fate was in doubt.
Kyl caught the White House by surprise when he said the Senate should not consider the treaty in Congress' postelection session, arguing that the pact would not get the consideration it deserved and Obama should wait until next year.
The administration had dispatched officials to Arizona in hopes of assuaging Kyl on one of his major concerns - budgeting adequate funds for the nation's nuclear arsenal and the laboratories that oversee them. The administration had pledged $80 billion to maintain the nuclear arsenal over the next 10 years, then added $5 billion more.
Early in December, a letter from the directors of the three major laboratories at Los Alamos, Lawrence Livermore and Sandia, in which they expressed satisfaction with the projected budget, broke the dam of opposition.
Some of that money is now in the pipeline, contained in a stopgap government funding bill that cleared Congress on Tuesday. The measure would finance the government, mostly at current levels, through March 4.
In announcing his support Wednesday, Sen. Lamar Alexander of Tennessee, the No. 3 Republican in the Senate, said he was reassured by a letter from Obama, in which the president reiterated his commitment to modernizing the remaining nuclear arsenal. A significant amount of that money would go to nuclear facilities at Los Alamos, N.M., and Oak Ridge, Tenn., a critical issue with Alexander and Sen. Bob Corker, R-Tenn.
During eight days of debate, Democrats turned back more than half a dozen Republican amendments that would have effectively killed the treaty.