Republican presidential candidates Donald Trump and Senator Ted Cruz looked to score support by opposing the Iran nuclear deal, while lawmakers began debating the agreement Wednesday in Congress.
Cruz called the agreement - forged between six world powers and Tehran - the most important issue in the 2016 presidential election.
"This Iranian nuclear deal is catastrophic," he said. "It is the single greatest national security threat facing America."
Cruz alleged that the agreement would allow Iran to both acquire a nuclear weapon and bankroll terrorist activities in the Middle East.
Several hundred opponents of the deal gathered outside the U.S. Capitol at a rally sponsored by the conservative Tea Party Patriots group, waving banners declaring "No Iran nukes" and "What part of death to America, death to Israel, don't you understand?"
"I've been doing deals for a long time," Donald Trump told the crowd. "Never ever, ever in my life have I seen any transaction so incompetently negotiated as our deal with Iran, and I mean never."
Democratic candidate Hillary Rodham Clinton told a Washington think tank earlier Wednesday that the deal was "not perfect."
"But it's a strong agreement. We absolutely should not turn it down," she said.
The former U.S. secretary of state promised to take military action if Iran tries to obtain a nuclear weapon in spite of the agreement.
Meanwhile, leaders in the lower House of Representatives looked set to postpone consideration of the deal amid disagreements within the conservative Republican Party about how to best voice opposition.
Cruz called for them to refuse to vote on the deal unless the Obama administration hands over separate agreements between the International Atomic Energy Agency and Iran.
Both the House and the Senate are weighing a resolution of disapproval of the deal, but President Barack Obama has secured support from enough lawmakers to preserve one of his key foreign policy achievements.
Both chambers of Congress have until September 17 to vote on a resolution of approval or disapproval of the deal.
Even if lawmakers pass a resolution against the agreement, Obama has vowed to veto any disapproval, and he has enough public support from Senate Democrats to prevent the Congressional override needed to scuttle the agreement.
Obama maintains the deal is the best way to prevent Iran from obtaining a nuclear weapon. Critics, including some in his own Democratic Party, say the controls do not go far enough and would allow Tehran to acquire a weapon once key provisions expire.
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