The accord to curb Tehran's program in exchange for sanctions relief has become a popular punching bag on the GOP presidential campaign trail, with one candidate, Texas Sen. Ted Cruz, threatening to tear it up on Day One of his administration.
At a Capitol Hill hearing, a top State Department official pushed back at the Republican criticism in an effort to keep its hard-won implementation on track.
Under Secretary of State Thomas Shannon told the Senate Foreign Relations Committee that if the next president withdraws from the deal, it could be calamitous.
"Any effort to step away from (the deal) would reopen a Pandora's box in that region that would be hard to close again," Shannon said.
A decision to exit the deal could trigger Iran's return to its nuclear weapons program, Shannon said, a possibility that many analysts say could lead to a nuclear arms race in the Middle East as Tehran's regional rivals, such as Saudi Arabia, work to match it.
A U.S. rejection of the deal "would be grasped by hardliners in Iran to assert that we were an unreliable interlocutor," Shannon said, and would be seen as "was a clear signal that they needed to return to their nuclear program."
With the deal in place, Shannon maintained, the pact "will allow for a program that is exclusively peaceful."
He also argued that stepping away from the deal would send ripples through close U.S. relationships as well.
"This would be an issue of concern" for those allies who negotiated the deal with Iran alongside the U.S., Shannon said, and would send a message that the U.S. isn't able to maintain consistent policies from one administration to another.
But some on the Republican side have charged that the deal itself is much more dangerous than the consequences of pulling out of it, despite the commitments the Obama administration has made.
Critics say that its time limits enable Iran to get a nuclear weapon in the next 10 to 15 years -- though the administration counters that without the deal, Iran could have gotten a nuclear bomb within months or a few years. Critics also point out that the agreement's enforcement mechanisms haven't prevented Iran from pernicious activity and that the deal has sparked public resistance in Israel, one of several skittish regional allies.
Cruz has said he would rip up the agreement reached between Iran, the U.S. and its allies on entering office, while Republican front-runner Donald Trump has called it "a disaster in virtually every way."
However, Ohio Gov. John Kasich, the other candidate seeking the GOP presidential nomination, has sounded a more moderate note, declining to say he would get rid of the deal.
Of the two candidates on the Democratic side, front-runner Hillary Clinton has taken the harsher tone, saying that she would "distrust and verify" and that she wouldn't support it if she felt "for one second" it put Israel in danger.
At the same time, she has pointed to the deal to demonstrate her experience as secretary of state.
Vermont Sen. Bernie Sanders, alternatively, said in July that the deal represented "a victory for diplomacy over saber-rattling."
Implementation of the nuclear pact began January 16 after the U.N.'s International Atomic Energy Agency certified that Iran was meeting all its obligations under the deal.
Senators on Tuesday also grilled Shannon about Iran's access to U.S. dollars in the wake of the deal, just hours after Secretary of State John Kerry said in an interview that it was "fair" for Iran to get what it "deserves" as it has held up its end of the pact.
Media reports had said the administration was considering easing financial restrictions that ban the use of U.S. dollars in trade with Iran. On Friday, President Barack Obama denied that, saying that the U.S. wasn't trying to facilitate dollar trade with Tehran.
Shannon repeated the point, saying the Obama administration has no plans to allow Iran access to the U.S. financial system or to facilitate transactions that would make it easier for the Persian Gulf nation to access U.S. currency.
Kerry was asked Tuesday morning on MSNBC why the administration approves of helping Iran get access to dollars or to get more investment.
"It is fair for Iran to get what it deserves because it kept its part of the bargain to date with respect to the nuclear agreement," Kerry said.
Shannon said the administration is trying to make sure that banks understand what they can and cannot do with and for Iran while remaining in line with continuing U.S. unilateral sanctions.
"As Iran attempts to access money that is being made available to it, there will be instances in which we will have to help Iran by clarifying regulations under which money will be available to them," Shannon said. "There are banks that are unclear about the regulatory structures and what sanctions have been lifted or not."