The debate on the Senate floor came hours after Senators Mitch McConnell of Kentucky and Jon Kyl of Arizona, the Republican leaders in the upper chamber, said they would vote against the treaty. While their opposition was not a surprise, the question was how aggressively Mr. McConnell in particular would lobby the handful of wavering Republicans who will decide the matter…

“I’m not going to vote for Start,” he added, “until I hear from the Russians that they understand we can develop four stages of missile defense, and if we do, they won’t withdraw from the treaty.”…

Never has a major nuclear arms control treaty been approved during a lame-duck session or without the support of the Senate minority leader. What makes the fierce showdown over this treaty so surprising is that compared with most of its predecessors, it is a relatively modest agreement that mainly resumes on-site inspections that lapsed last year and pares down each side’s deployed strategic warheads to 1,550 and deployed launchers to 700.

Republican critics have zeroed in on what they consider important flaws, including its verification program, the failure to address smaller, tactical nuclear bombs and some nonbinding language in the preamble that they argue would inhibit future American missile defense plans.