The Republican-led Congress is in the midst of a 60-day review of the deal, and is expected to vote in September on a resolution of disapproval that President Barack Obama has vowed to veto.
Schumer outlined his decision to vote against the deal in a statement posted on Medium.com late Thursday.
He said he had spent three weeks studying the deal and said advocates on both sides had made strong cases. He further praised President Barack Obama and Secretary of State John Kerry for their work to push Iran toward an agreement.
Still, Schumer said he had concluded that the country is better off without the deal. He said the deal had too many weaknesses, and he does not believe Iran will change its own policies.
He pointed specifically to the fact that inspections for nuclear restriction compliance could not be conducted without a 24-day delay – which would be enough time for Iran to hide what it might be doing with nuclear weapons.
Worse, Schumer said, the U.S. cannot demand inspections unilaterally under the deal. The majority of the eight-member Joint Commission must approve, and “it is reasonable to fear that, once the Europeans become entangled in lucrative economic relations with Iran, they may well be inclined not to rock the boat by voting to allow inspections.” (CBS New York)