Last night, New York Senator Chuck Schumer, the expected next Democratic Senate leader, announced his opposition to the recently-struck Iran deal. The move came as a major blow to persistent efforts by President Obama to cast the upcoming congressional vote on the deal as a partisan issue.

Other Democratic lawmakers, including former head of the Democratic Congressional Campaign Committee Rep. Steve Israel and Foreign Affair Committee member Rep. Ted Deutch, had previously come out against the deal. But Schumer’s opposition will make it extremely difficult to preset the debate in purely partisan terms, given the senator’s prominence within the Democratic party.

“Advocates on both sides have strong cases for their point of view that cannot simply be dismissed,” Schumer wrote in an extensive statement outlining his position. “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.”

After parsing many elements of the deal in careful detail, Schumer concluded by saying, “To me, the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great.”

Shortly after Schumer announced his opposition, he was joined by New York congressman Eliot Engel, the Democratic ranking member of the House Foreign Affairs Committee. “I still believe that a negotiated solution is the best course of action,” he said in a lengthy statement outlining his reservations. “That’s the path I believe we should pursue. But after careful consideration of all of the material; more than a dozen hearings since the beginning of the negotiating period; and conversations with Administration officials, experts, and many of my constituents, I regret that I cannot support this deal.”

Given the number of Democratic votes needed not just to pass a motion of disapproval on the Iran deal, but to achieve a two-thirds majority to override a presidential veto of the motion, it is exceedingly unlikely that Schumer’s opposition will tip the scales against the agreement. But if the angry reaction from White House allies and former staff members is any indication, it will make the Obama administration’s job more difficult in the days ahead.

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