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Republicans have conditioned their support for treaty ratification on two issues. First, Republicans have sought language in the ratification resolution clarifying that a bilateral committee set up under New START cannot discuss U.S. missile defenses except for the single issue covered in the treaty. The treaty prohibits using any more offensive missile silos for new defensive anti-missile interceptors.

Also, Sen. Jon Kyl, Arizona Republican, and other senators have conditioned support for the treaty on the administration’s commitment to following through with spending and programs to modernize the U.S. nuclear weapons arsenal over the next 10 years.

Mr. Obama has pledged to boost spending on this project by $10 billion over 10 years. The total for nuclear infrastructure upgrading will reach $80 billion.

Senate Minority Leader Mitch McConnell, Kentucky Republican, has said Republicans will follow the lead of Mr. Kyl and Sen. Richard G. Lugar, Indiana Republican and ranking member of the Foreign Relations Committee.

The committee passed by voice vote on Thursday a ratification resolution addressing these Republican concerns. The resolution, drafted by Mr. Lugar, contains a provision requiring reports from the Obama administration on the progress of the nuclear arsenal modernization, with language noting that the United States should withdraw from the treaty if the failure to upgrade the nuclear weapons system endangers U.S. security.

One Senate staff member who follows the issues, however, said Mr. Lugar’s resolution was not likely enough to persuade many Republican members to vote for the treaty. “Look at the vote in the committee,” this source said. “You had four votes against already.”

Passage of a treaty requires 67 votes in the Senate. Most arms control treaties have passed with no members voting in opposition.