The New Strategic Arms Reduction Treaty (START) has won ratification in the U.S. Senate, passing by a 71-26 vote. Ratification is a victory for President Barack Obama, those who seek a world with fewer nuclear threats as well as proponents of a constructive U.S.-Russia relationship.

New START was agreed to earlier this year — months behind schedule — and was signed by Mr. Obama and his Russian counterpart, President Dmitry Medvedev, in April. The treaty replaces the 1991 Strategic Arms Reduction Treaty, and limits each country to 1,550 deployed nuclear warheads (a reduction from 2,200 under an earlier agreement) and 800 strategic delivery vehicles. Approval by the Senate was delayed as U.S. politics descended into midterm turmoil and as Republican senators flexed their new muscle after the shellacking they administered to the Democrats in the November ballot.

The treaty enjoyed support from all senior members of the U.S. military establishment, as well as distinguished foreign policy experts and former officials. Nonetheless, serving Republicans were reluctant to endorse New START. Objections ranged from the spurious — insufficient time and attention given to discussion and debate — to the serious — whether the treaty might in some way limit U.S. freedom of maneuver regarding missile defense deployments and whether the U.S. nuclear infrastructure is being sufficiently maintained.

Administration officials insisted that the missile defense objection was unwarranted, arguing that the language in the preamble referring to missile defense was time-tested and that its intent was clear. In response to the second concern, the administration promised to request more than $85 billion over the next decade to build new nuclear research and production facilities and overhaul aging warheads.

That appears to have convinced holdouts. Once procedural votes were passed — credit considerable attention paid to (and arm-twisting applied to) GOP senators — the treaty gathered enough support to top the two-thirds majority needed for passage. In retrospect, it seems that GOP objections were in fact political: Republicans did not want to give Mr. Obama a political victory by approving the treaty. If this analysis is correct, then it says much about the toll partisanship is taking on U.S. politics and U.S. national security. In the past, Republican legislators led the fight for arms control treaties, and they were approved with lopsided majorities. The 1991 START agreement won Senate approval by a vote of 93-6, and the 2002 Moscow Treaty was ratified 95-0.

While some arms control supporters complain that New START does not go far enough — and their list of complaints is more substantive than that of the GOP — the real alternative — no treaty — would be much worse. New START is admittedly just a first step, but it restores momentum to the arms control process. It reduces the two superpowers’ nuclear arsenals and provides credibility to their claim that they are committed to disarmament. That commitment is instrumental to getting broader international buy-in for efforts to press for more effective nonproliferation measures and the creation of a unified front when dealing with governments like North Korea and Iran, suspected of being determined to acquire their own nuclear capabilities.

Equally significant is the boost that ratification gives to the U.S.-Russia relationship. Russian officials had made it clear that they saw the treaty as a litmus test for bilateral relations. Failure to approve the treaty would have been seen as a rejection of the U.S. administration’s attempt to “reset” relations with Moscow. While that relationship has been overshadowed in recent months by the U.S.-China relationship, Russia plays an important role in international politics. Moscow maintains considerable leverage throughout the world, especially in some particularly troublesome capitals. At a minimum, Russia can make trouble by refusing to support an emerging international consensus when dealing with those nations. New START is a reminder to leaders in Washington and Moscow that they share common interests and can make progress when they choose to work together.

Passage of New START is another legislative victory for Mr. Obama. Clearly, the president is not listening to critics who said he was supposed to see the midterm elections as a referendum on his tenure. Since the November ballot, he has secured passage of an economic package that nearly equals his original stimulus package in size, repeal of the “Don’t Ask, Don’t Tell” ban on gays serving in the military, stronger food safety regulation, extension of health benefits for 9/11 workers and ratification of New START. That is impressive for any legislative term, and especially for a “lame duck” session and a wounded president.

The lesson is that political tides shift quickly. Leadership can make a difference, when it is exercised in the genuine national interest. Mr. Obama remains a powerful force in U.S. — and world — politics when he can make the case that he is working on behalf of truly national interests and not purely partisan concerns.

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