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The rally, organized by the Tea Party Patriots, including Concerned Women for America, attracted hundreds outside the west lawn of the U.S. Capitol despite the hot weather.
"I've been doing deals for a long time ... that's what I do," Trump said. "Never ever, ever in my life have I seen any transaction so incompetently negotiated as our deal with Iran."
Trump went on to bash current American leadership -- "We are led by very very stupid people."
The GOP frontrunner promised he would bring home the four Americans - former FBI agent Robert Levinson, pastor Saeed Abedini, former U.S. marine Amir Hekmati and Washington Post's Jason Rezaian -- before he entered the White House.
"If I win the presidency I guarantee you those four prisoners are back in our country before I ever take office," Trump said. "I guarantee that."
Speaking immediately before Trump, Sen. Cruz called the Iran deal "catastrophic" and the "single greatest national security threat facing America."
"Stop this deal!" Cruz declared.
Cruz has been one of the few Republican presidential candidates who hasn't gone after Trump.
"I want to thank my friend Donald Trump for joining us today," Cruz said at the rally. The two embraced, shook hands and chatted briefly before Trump took the stage.
The joint appearance of Cruz and Trump at the rally was the latest friendly gesture between the GOP presidential candidates. The two have spent the summer appearing to cultivate what might be the friendliest relationship possible for two men both vying to be the next commander-in-chief.
Cruz has refused to say anything negative about Trump, even coming to his defense when Trump's comments on Mexican immigrants landed him in hot water.
Trump has publicly thanked Cruz on Twitter and said that he likes both Cruz and fellow presidential candidate Carson.
Cruz and Trump visited together for over an hour at Trump tower in July
During that visit, Cruz described the meeting between the two men as “terrific” and called Trump, “one of a kind.”
"He doesn't speak the way I speak, but I appreciate his highlighting the problem of illegal immigration,” said Cruz outside Trump Tower.
It was Cruz who invited Trump to participate in today's rally, but it appeared Trump seemed to overshadow Cruz.
For Cruz, it's the second day in a row he's shared the limelight with a fellow presidential candidate. Yesterday, he was at the same Kentucky rally that Mike Huckabee addressed with freed Rowan County Court Clerk Kim Davis. While Huckabee addressed the crowd from a stage, Cruz was standing in the crowd talking to reporters.
All the Democratic senators, as of yesterday, have declared where they stand on the deal. Currently 42 Democrats support the deal, while four oppose. Four previously undeclared Senate Democrats — Richard Blumenthal of Connecticut, Ron Wyden of Oregon, Gary Peters of Michigan and Maria Cantwell of Washington state — announced their support for the international agreement in a coordinated burst. That pushed supporters to a 42-vote total, one more than necessary to block a GOP disapproval resolution with a filibuster, and prevent a final vote.
"There is no better deal available now," declared Blumenthal, one of the Senate's Jewish Democrats, announcing his support for an accord that is strongly opposed by Israeli leaders as well as Republican senators.