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Critics of the system say that there is no proof it works and that the Iran threat is too distant. The Pentagon’s Missile Defense Agency estimates that Iran could develop a long-range missile capable of striking the United Sates by 2015.

“I don’t believe it is within the power of either McCain or Obama to promise anything, and it would be premature to do so, since the system doesn’t work,” said Daryl Kimball, executive director of the Arms Control Association.

Last week, Defense Secretary Robert M. Gates said former Soviet republic Lithuania is a “good alternative” to Poland as a site for the interceptors. Both countries are members of NATO, which formally endorsed the missile-defense project in April.

Russia has said the shield would pose a direct threat to its territory, an assertion the United States has repeatedly denied. Moscow, which rejected Washington’s proposal to cooperate, has threatened Poland and the Czech Republic with pointing Russian missiles at them if they agree to host the U.S. bases.