President Obama and Democratic Senator Chuck Schumer. Photo: Pete Souza/White House.

In a blow to President Obama’s foreign policy agenda, New York’s senior Democratic Senator Chuck Schumer announced his opposition to the Iran nuclear deal in a detailed and lengthy statement late Thursday night.

Weighing three separate aspects of the deal, Schumer, the 3rd highest ranking Democrat, said that on balance he believes a vote against the deal “is the right one.”

Schumer summed up his decision saying it was guided by his belief that “Iran will not change,” and therefore, under the agreement, “will be able to achieve its dual goals of eliminating sanctions while ultimately retaining its nuclear and non-nuclear power.”

Said Schumer, “Better to keep U.S. sanctions in place, strengthen them, enforce secondary sanctions on other nations, and pursue the hard-trodden path of diplomacy once more, difficult as it may be.”

Directly referencing President Obama’s assertion that opponents of the deal were akin to warmongers, the leading lawmaker said his opposition was “not because I believe war is a viable or desirable option, nor to challenge the path of diplomacy.”

U.S. Jewish groups on Thursday applauded the New York Democrat’s stance.

“The timing of Senator Schumer is especially important, in wake of President Obama’s speech wherein he in essence labeled Americans opposing the deal as warmongers and having a similar mindset as ‘death to America’ chanters,” said Simon Wiesenthal Center Associate Dean Rabbi Abraham Cooper. “Senator Schumer is a Democrats’ Democrat and his decision we hope will help open the minds and hearts of all undecided Senators and Congressmen to do the right thing.”

Others, however, were more skeptical of the Senator’s intent and suggested that behind the scenes deal-making had influenced his decision.

“If Schumer does, in fact, come out against the deal, it might mean that the future Dem leader understands that Obama’s victory is secure,” tweeted The Atlantic columnist Jeffrey Goldberg.

Jonah Goldberg, of the National Review, expressed a similar sentiment. “I hope I’m wrong, but my first assumption was that Schumer’s opposition to Iran deal means that Obama has the votes,” he tweeted.

Earlier on Thursday, Schumer’s colleague Senator Kirsten Gillibrand announced her support for the deal while calling it “imperfect,” and New York Congressman Eliot Engel voiced his opposition.

Many Jewish groups in the U.S. have been actively lobbying against the deal, saying it threatens the United States and puts Israel in unacceptable danger.