In a letter to Senate Minority Leader Mitch McConnell, a Kentucky Republican, Mr. Obama said that as long as he is president, the U.S. "will continue to develop and deploy effective missile defenses to protect the United States, our deployed forces, and our allies and partners."
President Obama's message was aimed at some Republican critics of the New START treaty who have argued that the pact with Russia would limit U.S. efforts to deploy missile-defense programs.
Senate Democrats deflected an attempt by Republicans on Saturday to strike a reference in the treaty's preamble to missile defense systems. Approval of the measure would have effectively killed efforts to ratify the treaty before the end of the year, because any changes to the agreement would force the U.S. and Russia to enter back into negotiations.
Still, 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
President Obama has made ratification of the treaty a top priority, and has urged lawmakers to approve the accord before the end of the year. The treaty has received the backing of scores of current and former military and national security officials, as well as former Republican President George H.W. Bush.
Defense Secretary Robert Gates has also backed Obama's assurances on missile defense. During an appearance at the White House on Thursday, Gates said the treaty "in no way limits anything we want or have in mind on missile defense."
The White House and Senate Democratic leaders have expressed confidence in prospects for ratification. The treaty requires a two-thirds vote in the 100-member Senate and its fate is uncertain.
Mr. Obama also used his weekly radio and Internet address Saturday to plead for Senate ratification.
"Ratifying a treaty like START isn't about winning a victory for an administration or a political party," the president said. "It's about the safety and security of the United States of America."
The treaty, signed by President Obama and Russian President Dmitry Medvedev in April, 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.
All 58 senators in the Democratic caucus are expected to back the treaty, but it needs Republican commitments to reach the necessary 67 votes.
"Every minute we drag our feet is a minute that we have no inspectors on the ground at those Russian nuclear sites," Mr. Obama said. "It's time to get this done."
Republicans used their weekly address to tell voters they "got the message" of the November midterm elections that will greatly increase the Republicans' ranks in Congress. Sen. John Cornyn, who heads the Senate Republican campaign committee, said his party is using its increased leverage to rein in government spending and prevent tax increases through the package signed by Obama on Friday that extends Bush-era tax cuts for all.
"This bipartisan agreement was made possible because voters gave Republicans much more leverage at the negotiating table," Cornyn said. "Our leverage forced the White House to abandon its `class-warfare' rhetoric, stop pandering to the president's left-wing base and do the right thing for American taxpayers and job creators."