WASHINGTON -- Sen. Jeff Flake, an Arizona Republican who President Barack Obama had been lobbying to support a proposed nuclear agreement with Iran, announced on Saturday that he was opposed to the deal.

Obama had hoped that Flake might be the only Republican to back the deal, giving the proposed agreement some kind of bipartisan support. Obama needs the support of at least 34 senators to sustain a presidential veto if Congress passes legislation to block the deal.

In a statement, Flake said that he opposed the agreement, called the Joint Comprehensive Plan Of Action, because he believed it restricted Congress' ability to impose new sanctions on Iran, despite assurances from the Obama administration.

While I have supported the negotiations that led to the JCPOA from the beginning, I cannot vote in support of this deal," Flake said in a statement. "The JCPOA does contain benefits in terms of limiting Irans ability to produce sufficient fissile material for a nuclear weapon for a period of time, particularly at its known nuclear facilities. But these benefits are outweighed by severe limitations the JCPOA places on Congress and future administrations in responding to Irans non-nuclear behavior in the region."

Twenty Senate Democrats so far have announced that they support the deal, which would require Iran to significantly reduce its nuclear capabilities in exchange for lifting economic sanctions. Critics say that the deal will allow Iran to use money from sanctions to fund terrorist groups and does not go far enough in limiting the country's nuclear capabilities. Secretary of State John Kerry has said that a better deal does not exist .

See a full list of the remaining senators who could make or break the deal here .

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