U.S. Senate Republicans on Saturday failed to amend an arms treaty with Russia, which they allege limits U.S. efforts to develop missile defense system.

The Democrats killed the amendment by a vote of 59 to 37 in the 100-member Senate, with four not voting. The amendment sought to strike language in the preamble to the treaty regarding the interrelationship between strategic offensive arms and strategic defensive arms.

Though the language in the preamble is not legally binding, leading Republican senators allege that the treaty could limit U.S. efforts to develop its missile defense system. Senator John McCain and others sponsored the amendment, which would mean renegotiations with Russia and killing of the treaty.

President Barack Obama on Saturday wrote to Senate minority leader Mitch McConnell and majority leader Harry Reid, assuring them that "the new START treaty places no limitations on the development or deployment of our missile defense programs."

He told them that "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."

In his weekly address on Saturday, Obama again argued for quick Senate approval of the new START treaty he signed in April with Russia President Dmitry Medevedev, stressing that "ratifying a treaty like START isn't about winning a victory for an administration or a political party, it's about the safety and security of the United States of America."

The pact 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.

"Every minute we drag our feet is a minute that we have no inspectors on the ground at those Russian nuclear sites," Obama said. "It's time to get this done."

The treaty is seen as a major achievement in foreign policy for the Obama administration. Obama is eager to see it pass the Senate this year, he has put off his scheduled departure on Saturday for Honolulu, Hawaii, for vacation in order to guide the treaty through before the holiday break.

The Senate is scheduled to continue debate on the treaty on Sunday for the fifth consecutive day.