The administration says that Russia has strong incentives to abide by the treaty because the U.S. arsenal is technologically superior and the costs of maintaining large stockpiles is harder for Russia to bear.
The defeat of the treaty would damage Obama's efforts to repair U.S.-Russian relations and to rally international cooperation on eliminating nuclear weapons.
Administration officials say that Republicans will ultimately come around because rejecting the treaty would leave the two countries dangerously uncertain about each other's arsenals. The authority to conduct inspections expired with the old START treaty last year.
"There is a simple question to ask: What is this and what if we don't have the treaty?" said Undersecretary of State Ellen Tauscher, the State Department's top arms control official. "I think that the risk of not having this is significant."