Washington (CNN) New York Sen. Chuck Schumer, an influential Jewish Democrat who's poised to assume leadership of his party in the Senate, will oppose President Barack Obama's nuclear deal with Iran, he announced on Thursday evening. 
 
 "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 wrote in a 1,600-word post on the website Medium. 
 
 "I will vote to disapprove the agreement, not because I believe war is a viable or desirable option, nor to challenge the path of diplomacy," he added later. "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." 
 
 Schumer's thinking on the deal has been closely watched since the plan to loosen sanctions on Tehran in exchange for access to potential nuclear sites was announced in early July. 
 
 Schumer would also vote to override the President's veto on Iran deal, an aide told CNN on Friday morning, meaning he is prepared to vote "no" twice against the President. 
 
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 Schumer’s opposition hurts the president’s efforts to woo the support of Senate Democrats. | AP Photo Iran deal takes hits from Chuck Schumer, GOP candidates Schumer's statement is published in the middle of the first GOP presidential primary debate. 
 
 New York Sen. Chuck Schumer dealt a blow to President Barack Obama’s nuclear deal with Iran, announcing during the Republican presidential debate Thursday night that he planned to vote against the agreement now under review in Congress. 
 
 Schumer’s opposition hurts the president’s efforts to woo the support of Senate Democrats, who he needs to be his bulwark against a likely Republican attempt to override a veto. But although Schumer said in a 1,600-word statement that he would side against the deal, he did not say whether he would vote to override a veto. 
 
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 “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,” Schumer said. An aide said Schumer would not only vote to disapprove of the deal, but would override a veto as well. 
 
 The Iran nuclear deal was also a big target during the Republican presidential debate. Scott Walker said he’d cancel it on Day One. Mike Huckabee said it left “hostages” behind. And Donald Trump said it was “a disgrace” for a president “who doesn’t have a clue.” 
 
 But while the candidates lobbed general criticisms at the deal, including that Obama gave away too much in the negotiations, none of the candidates offered a substantive alternative plan to curb Iran’s nuclear program. Like Schumer, Walker suggested that the next president should impose “even more crippling sanctions” on Tehran and then “convince our allies to do the same.” Analysts say that would be extremely difficult, given that the five other nations that joined in the nuclear deal with the U.S. and Iran are urging Congress to accept it, and warn that winning stronger terms isn’t a realistic option. 
 
 Indeed, the candidates seemed more interested in tossing off one-liners than proposing serious alternatives. For instance, Trump suggested that if Iran were a stock, “people should go out and buy it right now” because it will boom in the wake of the accord. 
 
 In a Wednesday speech about the Iran nuclear deal, Obama sought to portray critics of the nuclear deal unserious — he called the notion of major new concessions from Iran “a fantasy” — and cavalier about war. The Republican candidates showed little sign they had been chastened. Obama’s mockery of the deal’s critics, including a charge that Republicans were aligned with anti-deal Iranian hardliners, only seemed to embolden the GOP candidates. 
 
 While Ronald Reagan famously said “trust but verify,” Huckabee cracked, Obama’s motto is “trust but vilify.” 
 
 The former Arkansas governor also noted that the nuclear deal failed to secure the release of four Americans imprisoned or missing in Iran, whom he called “hostages.” Walker, meanwhile, cited the the 1979-1981 American hostage crisis in Iran as a reason why Iran “is not a place we should be doing business with.” 
 
 The Senate is set to begin debating a resolution to disapprove of the Iran agreement on Sept. 8, with a Sept. 17 deadline to vote on the measure. The measure is expected to pass both chambers, and Obama has said he intends to veto it when it reaches his desk. As a result, the final showdown in Congress is expected to be a vote to override that veto; the White House has expressed confidence that opponents will fail to get the two-thirds vote required in both the House and Senate. 
 
 Schumer’s formal opposition follows announcements from several Senate Democrats that they would support the deal, including independent Sen. Angus King of Maine, Tim Kaine of Virginia, and New York Democratic Sen. Kirsten Gillibrand. 
 
 Still, opposition from the third-ranking Senate Democrat — and the Senate Democratic leader-in-waiting in the next Congress — comes as little surprise. His announcement was followed by a similar once from fellow New York Democrat Eliot Engel, the ranking member on the House Foreign Affairs Committee, who also declared his opposition. 
 
 Though his announced opposition came as the political realm was preoccupied with the Republican presidential debate, Schumer had planned to make his position on the Iran deal official on Friday, according to a person familiar with the situation. 
 
 The New York senator had told the White House that he had decided to reject the nuclear agreement and would announce it on Friday. But the source accused the White House of leaking Schumer’s decision to the Huffington Post, forcing the senator to announce his decision Thursday night. 
 
 The White House did not immediately return a request for comment. 
 
 There had been growing signs that Schumer would oppose the nuclear agreement, with thousands of phone calls flooding his office and a million-dollar ad buy in New York City designed to pressure the Jewish senator. 
 
 Secure America Now, which has been working to secure opposition to the Iran deal, has organized more than 10,000 calls into Schumer’s Senate office, as well as 2,500 emails to key members of his staff and more than 275,000 engagements with his aides on Twitter. 
 
 Meanwhile, several other New York Democrats in Congress are also rejecting the Iran deal, such as Rep. Steve Israel, the influential former Democratic Congressional Campaign Committee chairman who announced his opposition earlier this week. Other Democratic lawmakers from New York, with a prominent Jewish population, are Grace Meng and Kathleen Rice — both House lawmakers. 
 
 Schumer’s opposition drew immediate criticism from liberals. MoveOn.org pledged that its 8 million members would withhold contributions from the Democratic Senatorial Campaign Committee and any Democratic candidate “who succeeds in undermining the president’s diplomacy with Iran.” 
 
 “Our country doesn’t need another Joe Lieberman in the Senate, and it certainly doesn’t need him as Democratic leader,” MoveOn.org Political Action executive director Ilya Sheyman said. “The vast majority of Democratic voters — the people who elected President Obama in part because of our shared belief that war must always be a last resort — will not stand for it.”
– Sen. Chuck Schumer, the top Jewish Democrat in Congress, has dealt President Obama's Iran deal a major blow but probably not a fatal one. The NY senator says that after "considerable soul-searching," he has decided to oppose the deal, which means Obama will have a harder task gathering 34 Senate votes to sustain a veto of any GOP legislation shooting down the deal, the New York Times reports. Schumer, who had been seen as a likely successor to Harry Reid as minority leader, says he is opposed "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." Lawmakers have until the middle of next month to make up their minds on the deal, and sources tell CNN that Schumer had been expected to delay announcing his opposition until Democrats were sure they had enough votes to keep the deal intact. The senator had been under heavy pressure to reject the deal, with the group Secure America Now organizing more than 10,000 calls to his office, reports Politico, which notes that the deal was discussed during last night's GOP debate. Donald Trump called it a "disgrace" and Scott Walker vowed to cancel it on his first day as president. (Obama told Congress this week that the price of rejecting the deal will probably be another war in the Middle East.)