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Ms. Tauscher said START represented the end of a new engagement process with Russia.

“I think that this process, this journey over the last year — President Obama and President [Dmitry] Medvedev, and Mr. Lavrov and Secretary [of State Hillary Rodham] Clinton setting the reset button — this has been a transformational exercise,” she said.

The undersecretary said the START process “is also the first exercise where we have worked cooperatively with the Russians on something of mutual benefit, where we have improved the relationship. In and of itself, the negotiation has improved the relationship.”

Steven Pifer, a senior fellow at the Brookings Institution and a former U.S. arms-control specialist, said the preamble to START will “acknowledge an interrelationship between offense and defense. But I also understand there are no limits on current or planned missile defense in the treaty itself.”

Mr. Pifer said he anticipates a “unilateral statement from the Russians” that would state more clearly the Russian understanding that changes in missile defense could prompt withdrawal. But Mr. Pifer said such a statement should not concern U.S. diplomats or senators.

“Who cares?” he said. “There is a supreme interest withdrawal clause that goes back to the original START treaty and can be invoked for any reason by either side so long as they provide six months’ notice.”

The bulk of START would limit the U.S. and Russian nuclear arsenals. The 1991 treaty reduced offensive weapons at the end of the Cold War to 6,000 warheads for each side. A subsequent 2002 agreement reduced arsenals to 2,200 warheads for each side. That agreement, however, had no verification component or counting rules. The new START would further limit offensive warheads to 1,550 for each side and is said to contain specific verification provisions.

Russia is thought to have no more than 2,600 active warheads, and the United States has 2,200 active warheads. The Union of Concerned Scientists, which counts each deployed missile as one warhead and each deployed bomber as one warhead, says the United States has 1,762 warheads and Russia has 1,741.

Mr. Bolton said the limits for the United States are too low.

“As I understand it, it is 1,550 for both parties. It is a mistake to have a limit equal to the Russians, given our global nuclear umbrella commitment,” Mr. Bolton said.

He pointed out that the U.S. umbrella extends to Australia, Japan and South Korea, as well as the countries in NATO.