But the 37-59 vote against the measure by Arizona Sen. John McCain exposes doubts about whether the Senate can ratify the treaty as written 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. Thirty-seven senators on Saturday voted for McCain's amendment to send the treaty back.
Republicans want to strike a reference in the treaty's preamble to missile defense systems. They say the preamble, as written, would allow Russia to withdraw from the pact if the U.S. develops a missile defense system in Europe.