Senate approval of President Barack Obama's nuclear arms treaty with Russia, which once looked close to a sure thing, is now in jeopardy.

The administration is scrambling to get enough Republican support in the Senate to ratify the New START treaty before the Democrats' majority shrinks by six in January. But Republicans have little incentive to give Obama a big political boost after leaving him reeling from their strong gains in last week's congressional elections.

A failure to win passage could trip up one of the administration's top foreign policy goals: improving relations with Russia. 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.

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.

A broader Republican fear is that the treaty is a small step toward weakening the U.S. nuclear deterrent. Though the treaty would only modestly affect the current U.S. arsenal, many Republicans see Obama's vision of a gradual elimination of the world's nuclear weapons as unrealistic. They argue that U.S. nuclear might is critical for American security and global stability.

The Obama administration is worried that ratification could slip out of reach if a vote were to be delayed. Ellen Tauscher, the undersecretary of state for arms control, said this week that the lame-duck session Congress will convene before most newly elected senators take their seats in January could be the administration's last shot.

"Our last opportunity to do it coming forward is in the lame duck," she said. "I think that, frankly, because of the way the numbers are working, it's the best opportunity to do it."

Republicans will gain one vote part of the way through the lame-duck session because they won a special election for Obama's former seat in Illinois. That will increase the GOP's Senate numbers temporarily to 42 in the 100-member chamber.

Since the election, senior administration officials, including Vice President Joe Biden and Secretary of State Hillary Rodham Clinton, have been pressing the case for ratification with Republican lawmakers. A long list of retired generals and senior statesmen from both parties have expressed support, arguing that that the treaty should be beyond politics.

But its best shot seems to lie in a political deal with one key Republican senator.

Republican Jon Kyl has wielded the most sway in his party on the issue. He has been negotiating with the administration for months and pinning support for the treaty to a boost in funding to modernize the U.S. stockpile of nuclear weapons. A number of his Republican colleagues have said they will follow his lead on the treaty. His approval could push support beyond the 67 votes the administration needs for ratification, although many Republicans still are likely to oppose it.

The administration has countered Kyl by warning that the lame-duck session also will be his last chance to get the money he is seeking for the nuclear stockpile because Democrats will not support him next year should the treaty fail.

The treaty would reduce the limit 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.

Treaty advocates have been warning that the United States has not had nuclear inspectors in Russia since December, when a previous treaty expired. They say Republicans who have opposed the treaty are endangering national security by delaying the inspectors' return.

Arlen Jameson, a retired Air Force general and former deputy chief of the Strategic Air Command, says a long delay in returning U.S. inspectors could force the U.S. military and intelligence agencies to find other ways to monitor Russian nuclear forces. He said that would involve costly monitoring by satellites that would shift scarce intelligence resources from operations in Iraq and Afghanistan.

"The needs for overhead monitoring is already under great stress," he said. "The expense will not be transparent because it will be not made publicly available, but it will be enormous."