Washington (CNN) New York Sen. Chuck Schumer, an influential Jewish Democrat who's poised to assume leadership of his party in the Senate, will oppose President Barack Obama's nuclear deal with Iran, he announced on Thursday evening.

"After deep study, careful thought and considerable soul-searching, I have decided I must oppose the agreement and will vote yes on a motion of disapproval," Schumer wrote in a 1,600-word post on the website Medium.

"I will vote to disapprove the agreement, not because I believe war is a viable or desirable option, nor to challenge the path of diplomacy," he added later. "It is because I believe Iran will not change, and under this agreement it will be able to achieve its dual goals of eliminating sanctions while ultimately retaining its nuclear and non-nuclear power."

Schumer's thinking on the deal has been closely watched since the plan to loosen sanctions on Tehran in exchange for access to potential nuclear sites was announced in early July.

Schumer would also vote to override the President's veto on Iran deal, an aide told CNN on Friday morning, meaning he is prepared to vote "no" twice against the President.

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