Sen. Ted Cruz (R-Texas). Reuters/Jim Bourg Sen. Ted Cruz (R-Texas) isn't backing off the tough rhetoric he's used to describe the Iran nuclear deal, despite criticism leveled by 2012 Republican presidential nominee Mitt Romney on Thursday.

Cruz has steadfastly maintained that President Barack Obama's administration would become a leading state-sponsor of terror if the agreement it struck with Iran makes it past Congress. He and others have argued that Iran would use a windfall from sanctions relief to finance terror abroad.

Both Obama and Romney have called Cruz's remarks inappropriate.

Romney wrote on Twitter early Thursday that he also opposes the Iran deal, but said Cruz's comments were "way over the line on the Obama terrorism charge. Hurts the cause."

Cruz, 2016 presidential candidate, fired back at Romney in a Thursday radio interview with KFYO's Chad Hasty.

"So Mitt Romney's tweet today said, 'Gosh, this rhetoric is not helpful,'" Cruz said. "John Adams famously said, 'Facts are stubborn things.' Describing the actual facts is not using rhetoric; it is called speaking the truth."

The senator recalled what he described as a critical moment during the 2012 presidential race: A back-and-forth over that year's attack on a diplomatic facility in Benghazi, Libya.

"Part of the reason that Mitt Romney got clobbered by Barack Obama is because we all remember that third debate where Barack Obama turned to Mitt and said, 'I said the Benghazi attack was terrorism and no one is more upset by Benghazi than I am.' And Mitt, I guess listening to his own advice, said, 'Well gosh, I don't want to use any rhetoric. So OK, never mind. I'll just kind of rearrange the pencil on the podium here,'" Cruz said.

He added that the 2016 presidential candidates need to speak up or they will fail like Romney.

"We need to stand up and speak the truth with a smile," he said. "The truth has power and every time we have Republicans who shy away — who don't want to engage, who don't want to speak the truth — we lose."

Mitt Romney vs. Barack Obama. John Moore / Getty

The nuclear deal — struck earlier this month among the US, Iran, and other world powers — grants billions of dollars of sanctions relief in exchange for Tehran curbing its nuclear ambitions. Republican critics like Cruz, who hope to defeat the agreement in Congress, argue that Iran will use the cash windfall to sponsor terror in places like Syria, Lebanon, and Yemen.

During his Thursday radio interview, Cruz also connected his controversial terrorism argument to former Secretary of State Hillary Clinton, who served in the Obama administration and is the Democratic presidential front-runner in 2016.

"Those billions of dollars in American control will be used by jihadists to murder Americans, to murder Israelis, to murder Europeans. Those are the facts," he said. "The unavoidable consequence of those facts is that if this deal goes through, Barack Obama, and Hillary Clinton, and John Kerry, will be leading global financiers of radical Islamic terrorism on the face of the earth."

Cruz added: "When you send billions of dollars to jihadists trying to kill Americans, you bear responsibility for the murder that they carry out with the money you have given them."