Sen. Charles Schumer Chuck SchumerRepublican senator says plans to confirm justice before election 'completely consistent with the precedent' Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year MORE, the heir apparent to the role of Senate Democratic leader, late Thursday evening announced he opposes President Obama's nuclear deal with Iran.

Schumer’s decision — revealed while the political world was focused on the first GOP presidential debate — is a serious blow to the Obama administration, which had been gaining significant momentum in support of the Iran deal among Senate Democrats in recent days.

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“Advocates on both sides have strong cases for their point of view that cannot simply be dismissed,” Schumer, the No. 3 Senate Democrat, wrote in a post on 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.”

“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.”

Making matters worse for Obama, the top Democrat on the House Foreign Affairs Committee, Rep. Eliot Engel (D-N.Y.), also came out against the deal Thursday night.

Both supporters and opponents of the Iran deal had been intently following Schumer, who has long pursued a pro-Israel foreign policy but has been under intense pressure to back the Iran deal from the leaders of his party. Many had been expecting that he would not announce a decision until shortly before Congress's September vote on the agreement, after other lawmakers had already made their positions known.

But The Huffington Post reported that Schumer was planning to oppose the deal earlier on Thursday evening, making the break weeks before the expected vote on the Iran deal in the Senate.

Engel, meanwhile, said in a statement that he “cannot support” the deal.

"I still believe that a negotiated solution is the best course of action,” he said. “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," Engel added.

The opposition of Schumer could be devastating to the Obama administration, which has been vigorously lobbying congressional Democrats to get behind the Iran deal, expecting virtually every Republican lawmaker to oppose it. If GOP lawmakers vote as a bloc, Obama would need Democrats to have his back to sustain his veto of legislation to kill the deal.

It remains to be seen how hard Schumer will work to convince his colleagues to oppose the deal. Should he mount a vigorous lobbying effort against it, Senate Democrats will be torn between Obama and the man that many hope will lead them back to the majority.

“There are some who believe that I can force my colleagues to vote my way,” Schumer said a separate statement Thursday evening. “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."

While the moves by Schumer and Engel were overshadowed somewhat by the GOP debate, the twin statements effectively upend the White House’s narrative about its lobbying efforts, and cast a shadow over its hopes for securing support from an overwhelming number of congressional Democrats.

The news of the two lawmakers’ thinking came mere hours after their fellow New York Democrat, Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D), came out in support of the Iran deal.

- This story was last updated at 12:16 a.m.