WASHINGTON — Sen. Chuck Schumer, a New York Democrat expected to be the party’s next leader in the Senate, said he will oppose the Iran nuclear deal in spite of President Barack Obama’s intense lobbying in favor of the accord.

The deal, struck last month with Tehran and Western powers, would curb Iran’s nuclear program in exchange for billions of dollars in relief from crippling sanctions.

“The very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great,” Schumer said in opposing the pact. He said he based his decision on the nuclear and non-nuclear elements of the accord and on the question, “Are we better off with the agreement or without it?”

A leading Jewish Democrat, Schumer was the first senator of Obama’s party to step forward to oppose the deal. His announcement Thursday night came just hours after two other Senate Democrats — New York’s Kirsten Gillibrand and New Hampshire’s Jeanne Shaheen — announced their support for the international accord.

“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,” he said in a statement issued weeks before he will cast a vote.