WASHINGTON — In a blow to President Barack Obama, chances faded Tuesday for Senate approval of a major nuclear arms treaty with Russia this year, tripping up one of the administration's top foreign policy goals: improving relations with Moscow.

The administration reacted swiftly with Vice President Joe Biden warning that Senate failure to ratify the treaty would endanger the national security of the United States.

He said without ratification, Americans will have no way to verify Russia's strategic nuclear arsenal.

Biden said it would weaken cooperation between two nations that hold 90 percent of the world's nuclear weapons.

Obama has been pushing to get enough Republican support for a vote before the Democratic majority shrinks by six in January. Just over the weekend, he had expressed optimism on the prospects of the treaty, perhaps his most significant foreign policy achievements.

The administration is worried that ratification could slip out of reach if a vote were delayed until next year. Even with their current large majority in the Senate, Democrats have struggled to win enough support from Republicans, many of whom have opposed the treaty or raised concerns.

At a minimum, that would likely set the treaty back for months, because Republicans are likely to demand new hearings in the Senate Foreign Relations Committee so that newly elected lawmakers are briefed.

Last week the administration sought to satisfy Kyl's conditions for supporting the treaty with a proposal to significantly boost funding for the nation's nuclear weapons complex. A congressional aide briefed on White House plans told The Associated Press last week that the White House was proposing to add $4.1 billion that would go to maintaining and modernizing the arsenal and the laboratories that oversee that effort. U.S. government officials traveled to Kyl's home state of Arizona to make the proposal.

In his statement Tuesday, Kyl suggested that the offer had not resolved the issue.

"When Majority Leader Harry Reid asked me if I thought the treaty could be considered in the lame duck session, I replied I did not think so given the combination of other work Congress must do and the complex and unresolved issues related to START and modernization," he said.

State Department spokesman P.J. Crowley said that the administration believes that it has already addressed the concerns of Kyl and other Republicans on the treaty and funding for the nuclear stockpile.

"It remains the Obama administration's belief that the New START treaty is in our national interest and we believe it should be voted upon in this lame duck session," Crowley said.

A lame duck session occurs when the outgoing Congress meets after an election to try to clear up unfinished business before its incoming successor body convenes in January.

Kyl said that he had spoken with Reid Monday night. The Republican senator's support is crucial because a number of his Republican colleagues have said they will follow his lead on the treaty. So his approval could push support beyond the 67 votes the administration needs for ratification.

Kyl has maintained that boosting funding for the stockpile would ease Republican concerns about the treaty by demonstrating that the administration is serious about maintaining a robust U.S. nuclear deterrent. The treaty would reduce U.S. and Russian limits on strategic warheads to 1,550 for each country from the current ceiling of 2,200. It also would set up new procedures to allow both countries to inspect each other's arsenals to verify compliance.

Some Republicans have argued that the treaty would limit U.S. missile defense options and does not provide adequate procedures to verify that Russia is living up to its terms. Advocates dispute both charges.

The treaty, signed in April by Obama and Russian President Dmitry Medvedev, has been the most tangible sign of success, and failure to get it ratified could be viewed as a rebuke in Moscow. It also would leave Obama's push for even greater restrictions on the world's nuclear arsenal in doubt.


Associated Press writer Donna Cassata contributed to this report.