Appearing during an hours-long rally on the Capitol lawn where speaker after speaker railed against the deal, Republican leadership and President Barack Obama, Cruz warned that the Obama administration would become "the leading financier of radical Islamic terrorism" if he deal goes through.
"It's worth remembering that if this deal goes through we know to an absolute certainty that people will die," Cruz said.
Trump, who took the stage to REM's "It's the End of the World as We Know It," said that he has "been doing deals for a long time" but has never seen one like the Iran accord.
"I've been making lots of wonderful deals, great deals, 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. And I mean never."
Trump had choice words for Obama and Congressional leadership.
"We are led by very, very stupid people. Very, very stupid people. We cannot let it continue," Trump told crowd, eliciting large cheers. He said America "can't beat anybody" and vowed that would change if he is elected president.
"We will have so much winning if I get elected that you may get bored with winning. Believe me!" Trump said. "You'll never get bored with winning."
Trump and Cruz, who despite being rivals for the Republican presidential nomination enjoy a relatively cozy relationship, were joined by former Alaska governor Sarah Palin, conservative radio hosts Glenn Beck and Mark Levin, members of Congress and others on the West Lawn of the Capitol. Cruz helped organize the event, along with the Tea Party Patriots and the Zionist Organization of America.
Cruz said he hoped that Senate Democrats would change their mind on the deal- that they would "fall to their knees and pray" and decide to change their vote — but if they support the deal they will bear responsibility if people die because of it.
"I want to ask every Senate Democrat, 'How will you look in the eyes of the mother or father or sons or daughters of those who are murdered by jihadists, those Americans who are blown up, those American who are shot, those Israelis who are murdered?'" Cruz said. "And let me be clear if you vote to send billions of dollars to jihadists who pledge to murder Americans, then you bear direct responsibility for the murders carried out with the dollars you have given."
He added: "You cannot wash your hands of that blood."
Despite being rivals for the Republican presidential nomination, Cruz and Trump enjoy an unusually cozy relationship. Cruz, who invited Trump to the rally because he would bring the spotlight, praised the real estate mogul as "my friend" and the two men embraced on stage.
"Donald has an incredible ability to attract attention," Cruz told reporters after the rally.
Hundreds of people stood in sweltering heat on the west lawn of the Capitol for the rally; many huddled under a large tree far from the stage to shade themselves from the blazing sun. Many carried signs; some read "#JewishLivesMatter ask God" and "Give them hell Ted Cruz." People waved huge American and Israeli flags. A group of bagpipers played patriotic songs. People sported shirts sporting the American flag and a contingent of men dressed in colonial garb showed up. A woman from Phoenix held a punching bag with a cartoon of Obama on it; a man walked up and asked if he could hit it.
"Can I punch him, please?" the man, Jim Scearce of Danville, Virginia, asked, and he hit the bag. "My only dream is that could have been real life."
Barb Bullock of Harrington, Delaware — "where the casino is," she said — waved an American flag with a print of the Statute of Liberty and words "Freedom Isn't Free" on it and a Tea Party windsock. She said she wanted to send a message to Sen. Chris Coons, D-Del., who is supporting the deal.
"I don't want them to be nuclear," she said.
The woman holding the punching bag, Rose Prescott of Phoenix, was on vacation in Virginia and made sure to attend the rally.
"We don't trust Iran. They are liars. Their goal is to take out Israel," she said, adding she attended "To tell Congress to grow a pair and do what the people want."
The crowd consistently yelled "Amen!" and booed any mention of Obama, Democrats, House Speaker John Boehner, R-Ohio, and Senate Majority Leader Mitch McConnell, R-Ky.
In a winding speech that included references to Russia being near Alaska and President Obama's selfie stick and Hillary Rodham Clinton, Palin said the U.S. is "suppin' with Sharia" with the deal.
"Only in an Orwellian Obama world full of sprinkly fairy dust blown from atop his unicorn as he's peeking through a really pretty pink kaleidoscope would he ever see victory or safety for America or Israel in this treaty," Palin said.
Forty-two Democratic senators had come out in favor of the deal as of Tuesday, enough to prevent a resolution opposing the agreement to reach Obama's desk. White House press secretary Josh Earnest said the administration feels "gratified" by the development and suggested that Democrats who support the deal filibuster legislation against it.
"We certainly would expect that those members of Congress who support the agreement to take the necessary steps in Congress to prevent Congress from undermining the agreement," Earnest said Tuesday.
The deal has been fiercely opposed by Jewish groups, evangelical Christians, conservatives and others, including Democrats and Republicans in Congress. Despite the momentum toward passage, organizers said they intended for the rally to galvanize opposition to the agreement and potentially sway legislators who are on the fence.
Trump has blasted the deal as "incompetent," "insane" and "horrible." Cruz has made repealing the Iran deal one of his signature issues on the campaign trail, calling it the "single greatest national security threat facing America" and vowing to undo the deal on his first day in office, should he be elected president.
Cruz was rebuffed when he tried to tie a bill imposing additional sanctions on Iran to a highway bill, and threatened in July to block funding for the State Department if the Senate did not vote. Cruz said the Obama administration would become the world's leading terrorism financier if the deal were to go through.
Cruz took to the Senate floor Wednesday morning before the rally to rail against the deal.
"This terrible deal will not stop a virulently anti-American and anti-Israeli regime from getting a nuclear bomb," he said.
In a speech last week, Cruz likened the deal to the movie "Scarface," equating a provision that would let Iran monitor its facilities to allowing the movie's main character, drug dealer Tony Montana, to investigate his empire.
"Have any of y'all seen the movie 'Scarface?'" Cruz asked, according to CNN. "This is the equivalent of law enforcement picking up the phone and calling Tony Montana and saying, 'Hey Tony, you got any drugs?' 'I don't got no drugs.' 'Thank you, Tony.' That is essentially the Iranian nuclear inspection regime."
Louis Morris, 71, of Silver Spring, Maryland, said he knows that Democrats have enough votes, but he wanted to send a message that people are against the deal — and possibly change some minds.
"It ain't over until it's over," Morris said.
