Senate Democrats deflected an initiative by Republicans on Saturday that would have forced U.S. and Russian negotiators to reopen an arms treaty reducing stockpiles of nuclear warheads.
But the 37-59 vote against an amendment by Sen. John McCain, R-Ariz., exposed doubts about whether President Barack Obama can win Senate ratification of the treaty before a new, more Republican Congress assumes power in January.
Treaties require a two-thirds majority of those voting in the Senate, or 67 votes if all 100 senators vote.
Led by McCain, Obama's GOP opponent in the 2008 presidential election, Republicans tried to strike words from the treaty's preamble that they say would allow Russia to withdraw from the pact if the U.S. develops a missile defense system in Europe.
The treaty is a foreign policy priority for Obama, who signed it in April with Russian President Dmitry Medvedev.
It would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200, and establish a system for monitoring and verification. U.S. weapons inspections ended a year ago with the expiration of the 1991 arms control treaty.
Obama used his weekly radio and Internet address Saturday to call for ratification.
He also tried to allay GOP doubts with a letter Saturday to Senate Minority Leader Mitch McConnell, R-Ky., pledging to carry through with planned U.S. missile defense facilities in Romania and Poland that would be capable of intercepting a missile from Iran aimed at the U.S.
As long as he is president, Obama said, the U.S., "will continue to develop and deploy effective missile defenses to protect the United States, our deployed forces, and our allies and partners."
The treaty has received the backing of current and former military and national security officials, as well as former Republican President George H.W. Bush.
"Ratifying a treaty like START isn't about winning a victory for an administration or a political party," the president said. "It's about the safety and security of the United States of America."
Democrats said a reference in the treaty's preamble on missile defense systems is nonbinding and has no legal authority. In his letter, Obama said the U.S. disagrees with Russian statements about the threat that a missile defense poses to the strategic balance between the two countries.
"If you change it, it requires this treaty to go back to the Russian government, and then we don't have any treaty," said Sen. John Kerry, D-Mass., chairman of the Senate Foreign Relations Committee. Republican critics said Russia is using the treaty to continue its opposition to the deployment of a U.S. missile defense shield.
"Russia is trying to force the United States to choose between missile defense and the treaty," said Sen. John Barrasso, R-Wyo., who co-sponsored McCain's amendment. "In that case I choose missile defense."