WASHINGTON (AFP) - The US Senate on Saturday beat back an attempt by President Barack Obama's Republican foes to kill a nuclear arms control pact with Russia over charges it may cripple US missile defense plans. Lawmakers voted 37-59 to reject an amendment by Republican Senator John McCain to strip out language in the new Strategic Arms Reduction Treaty's (START) preamble tying offensive nuclear weapons to defensive systems. The preamble is non-binding but, because it resulted from talks between Washington and Moscow, passing the amendment would have forced the accord back to the negotiating table, effectively killing the agreement. Just before the vote, the White House released a letter from Obama to top lawmakers reaffirming his plan to deploy US missile defense systems and rejecting Russia's claim that doing so would justify withdrawing from START. "Regardless of Russia's actions in this regard, as long as I am president, and as long as the Congress provides the necessary funding, the United States will continue to develop and deploy effective missile defenses to protect the United States, our deployed forces, and our allies and partners," he said. Obama's strong message on an issue that has at times deeply angered Moscow came in a letter to Democratic Senate Majority Leader Harry Reid and his Republican counterpart, Republican Senator Mitch McConnell. US Defense Secretary Robert Gates and top uniformed US military commanders have repeatedly rejected Republican charges that the accord hamstrings US missile defense plans -- a message Obama echoed in his letter. "The New START treaty places no limitations on the development or deployment of our missile defense programs," the president said. Three Republicans voted against the amendment, while Independent Senator Joe Lieberman backed it -- but he is expected to back ratifying the treaty. Opponents of the accord have highlighted the preamble's affirmation of an "interrelationship between strategic offensive arms and strategic defensive arms" -- an unmistakable reference to US missile defense efforts. "This interrelationship will become more important as strategic nuclear arms are reduced," and "current strategic defensive arms do not undermine the viability and effectiveness of the strategic offensive arms of the parties," the preamble says. Republicans have also pointed to Russia's unilateral statement, when the treaty was signed by Obama and Russian President Dmitry Medvedev in April, that a rise in US missile defenses could lead Moscow to back out of the accord. Russia said reasons to quit the treaty under a clause saying that either side may do so if their national security is threatened "include a build-up" of US missile defenses "such that it would give rise to a threat to the strategic nuclear force potential of the Russian Federation." "The United States did not and does not agree with the Russian statement," Obama said in his letter, which the White House made public. Republicans appeared to be caught off guard by the letter, which Democratic Senator John Kerry read to his colleagues, though McCain responded: "Presidents don't last forever, but binding treaties do." Republicans have introduced other treaty amendments -- but they control just 42 seats in the 100-seat Senate and 51 votes are needed to approve modifying the accord, meaning they are all-but-certain of being defeated. The agreement -- which has the support of virtually every present and past US foreign policy or national security heavyweight -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002, and 800 launchers and bombers. The accord would also return US inspectors who have been unable to monitor Russia's arsenal since the treaty's predecessor lapsed in December 2009. Obama won a critical victory when lawmakers voted 66-32 Wednesday to take up the pact, showing Democrats within striking distance of the 67 votes needed to ratify START if all 100 Senators are present. One of those absent, Democratic Senator Evan Bayh, has pledged to back the agreement.