Sen. Chuck Schumer, one of the most influential Jewish Democrats in Congress, came out in opposition to President Obama’s nuclear deal with Iran last night.

“Advocates on both sides have strong cases for their point of view that cannot simply be dismissed,” Schumer, D-N.Y., wrote in a post on the site Medium. “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.”

Schumer joined a number of top congressional Democrats speaking out against the Obama administration’s deal with Iran. Following Senate Minority Leader Harry Reid’s decision to retire in 2016, many believe that Schumer is poised to become his party’s leader.

In his post on Medium, Schumer praised Obama and Secretary of State John Kerry for their work in the negotiations, as well as for “combating and containing Iran.”

However, the New York Democrat, who spent weeks studying the plan, criticized specific components of the Iran deal, including the terms of inspections and the provisions surrounding the ability to reimpose sanctions on Tehran.

Schumer said his biggest concern regarding the deal is that after 10 years, many of the restrictions against Iran are removed.

“If Iran’s true intent is to get a nuclear weapon, under this agreement, it must simply exercise patience,” Schumer said. “After 10 years, it can be very close to achieving that goal, and, unlike its current unsanctioned pursuit of a nuclear weapon, Iran’s nuclear program will be codified in an agreement signed by the United States and other nations. To me, after 10 years, if Iran is the same nation as it is today, we will be worse off with this agreement than without it.”

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Members of Congress have until mid-September to decide whether they will support or oppose the deal.

President Obama has signaled he would veto any measure opposing the Iran deal. But Republicans—the majority of whom oppose the deal—are also working to secure votes from Democrats to override a presidential veto.

Schumer’s office said yesterday the senator would also vote to overcome the veto. The president needs 34 votes to sustain his veto.

So far, 12 Senate Democrats and Sen. Angus King, I-Maine, have come out in support of the president’s deal with Iran.

However, Rep. Eliot Engel, D-N.Y., the ranking member of the House Foreign Affairs Committee, announced his opposition yesterday. Engel is a top Democrat in the House and, like Schumer, is an influential Jewish figure.

Other Democratic opponents of the deal in the House include Reps. Steve Israel of New York, former chair of the Democratic Congressional Campaign Committee, Nita Lowey of New York, ranking Democrat of the House Appropriations Committee, and Rep. Ted Deutch of Florida, ranking member of the House Middle East and North Africa subcommittee.

“I will vote to disapprove the agreement, not because I believe war is a viable or desirable option, nor to challenge the path to diplomacy,” Schumer said. “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.”