Barack Obama urges US Senate to pass new Start treaty
President Barack Obama has renewed his call for US senators to ratify an arms control treaty with Russia before the Democratic-led Congress breaks up.
In his weekly address, Mr Obama said the "safety and security of America" was at stake.
Ratification of the new Strategic Arms Reduction Treaty (Start) has been delayed by disagreements.
But a Republican attempt to force Russia and the US back into talks on the wording of the treaty has failed.
The treaty, which was signed by Mr Obama and President Dmitry Medvedev in April, needs a two-thirds majority in the Senate to become law.
That means that the Democrats need two independents and a number of Republicans to vote in favour of ratification.
But Republicans have raised a number of concerns, including whether the treaty would allow the modernisation of America's nuclear arsenal.
Senator Kit Bond, the most senior Republican on the Senate Intelligence Committee, condemned the treaty as "giving Russia essentially a vote on our missile defence decisions".Amendment rejected
In his radio address, Mr Obama appealed to the Senate to put aside partisan differences and ratify the treaty.
Failure to do so would hit US credibility in the eyes of other nations, he said.
"We'll risk undermining American leadership not only on nuclear proliferation, but a host of other challenges around the world.
"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."
Later on Saturday the Senate voted against a Republican amendment which, if passed, would have ruled out the treaty's ratification by the end of the year.
Republicans had been seeking to strip out words in the preamble to the treaty referring to missile defence - a move which would have meant forcing the US and Russia back into negotiations.
Just before the vote, the White House released a letter from Mr Obama to leading senators which sought to address Republican concerns.
It insisted that as long as he was president, the US would "continue to develop and deploy effective missile defences to protect the United States, our deployed forces, and our allies and partners."Warhead reduction
The treaty is a key part of Mr Obama's efforts to "reset" relations with Russia.
Its terms would restrict each nation to a maximum of 1,550 deployed nuclear warheads - a cut of about 30% from a limit set eight years ago.
It would limit to 700 the number of deployed ballistic missiles or nuclear bombers.
It would also establish a new mechanism for sending inspectors to the other country's nuclear sites.
Since the previous Start treaty expired in December 2009, Russia and the US have not been able to conduct inspections of each other's nuclear stockpiles - leading to uncertainty about what the other side is doing.
A vote is expected early next week.