WASHINGTON — Senator Chuck Schumer, the most influential Jewish voice in Congress, said Thursday night that he would oppose President Obama’s deal to limit Iran’s nuclear program.

“Advocates on both sides have strong cases for their point of view that cannot simply be dismissed,” Mr. Schumer, Democrat of New York, said in a lengthy statement. “This has made evaluating the agreement a difficult and deliberate endeavor, and 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.”

Mr. Schumer had spent the last several weeks carrying a dog-eared copy of the agreement in his briefcase and meeting with Mr. Obama and officials like Wendy R. Sherman, the deal’s chief negotiator. With his decision, he paves the way for other Democrats on the fence to join Republicans in showing their disapproval.

“There are some who believe that I can force my colleagues to vote my way,” Mr. Schumer said. “While I will certainly share my view and try to persuade them that the vote to disapprove is the right one, in my experience with matters of conscience and great consequence like this, each member ultimately comes to their own conclusion.”