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Iranian women chant slogans during an anti-US demonstration outside the former US embassy headquarters in the capital Tehran on May 9, 2018. - Iranians reacted with a mix of sadness, resignation and defiance on May 9 to US President Donald Trump's withdrawal from the nuclear deal, with sharp divisions among officials on how best to respond.
For many, Trump's decision on Tuesday to pull out of the landmark nuclear deal marked the final death knell for the hope created when it was signed in 2015 that Iran might finally escape decades of isolation and US hostility. (Photo by ATTA KENARE / AFP)        (Photo credit should read ATTA KENARE/AFP/Getty Images)
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Iranian women chant slogans during an anti-US demonstration outside the former US embassy headquarters in the capital Tehran on May 9, 2018. - Iranians reacted with a mix of sadness, resignation and defiance on May 9 to US President Donald Trump's withdrawal from the nuclear deal, with sharp divisions among officials on how best to respond. For many, Trump's decision on Tuesday to pull out of the landmark nuclear deal marked the final death knell for the hope created when it was signed in 2015 that Iran might finally escape decades of isolation and US hostility. (Photo by ATTA KENARE / AFP) (Photo credit should read ATTA KENARE/AFP/Getty Images)
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Story highlights

Jeff Flake was seen by the Obama administration as a potential yes vote for nuclear proposal

The Arizona senator has split from party before on some foreign policy matters

Flake says he thinks Congress is too limited by the potential deal and wouldn't be able to impose sanctions

(CNN) —  

Sen. Jeff Flake announced Saturday that he will vote against the proposed nuclear agreement with Iran.

The White House and Democrats had hoped the Arizona Republican would support the potential deal.

Flake said in a statement released on his website that the Joint Comprehensive Plan of Action, as it is called, includes requirements he views as positives.

“But these benefits are outweighed by severe limitations the JCPOA places on Congress and future administrations in responding to Iran’s non-nuclear behavior in the region,” he said.

Flake, a member of the Senate Foreign Relations Committee, has broken with his party on foreign policy matters in the past. And he was the only Republican member of Congress to go to Africa with the President.

But the senator said he was concerned the White House wouldn’t confront Iran on its interpretation of the proposal and therefore would be reluctant to challenge Tehran on is activities in the Mideast. He also believes the President’s assurances that the deal doesn’t affect Congress’ ability to impose sanctions on Iran aren’t confirmed by the text of the agreement.

“Hoping that Iran’s nuclear ambitions might change after a 15-year sabbatical might be a bet worth making. Believing that Iran’s regional behavior will change tomorrow – while giving up tools to deter or modify such behavior – is not,” Flake said in his statement.

Sen. Jeff Flake, R-Ariz.
Drew Angerer/Getty Images
Sen. Jeff Flake, R-Ariz.

Flake’s announcement comes as more Democrats late this week signaled they would back the agreement. On Thursday, Minnesota Sen. Al Franken, who is Jewish, voiced his support for the deal in a CNN op-ed. Montana Sen. Jon Tester also said he would support the deal on Thursday.

RELATED: Al Franken: Why I support Iran deal

Although the loss of Flake likely means no Republicans will vote for the agreement, the White House is confident it can retain the necessary Democratic support to override a potential veto in the Senate.

So far, the GOP only has one solid Iran detractor among Democrats in the Senate, as New York Sen. Chuck Schumer expressed his opposition to the deal earlier this month. Senate Minority Leader Harry Reid said Wednesday he is still undecided.

RELATED: More lawmakers pick sides on Iran nuclear deal