But when he declares that it has not been in US interests, he will consign the proudest legacy achievement of President Barack Obama's second term to a deeply uncertain future -- and could even set off a train of consequences that could eventually lead to its collapse.

Should that be the case, Trump, or one of his successors in the Oval Office, may one day face the fateful choice that the deal was supposed to circumvent -- whether to use military force to stop the Islamic Republic racing toward the bomb.

The President has fumed against what he has called a "very bad deal" and an "embarrassment" to the country despite all available evidence that Iran is complying with terms which imposed limits on its nuclear program in return for a lifting of sanctions that had crippled its economy.

"I think it was one of the most incompetently drawn deals I've ever seen," Trump told Fox News' Sean Hannity on Wednesday.

Trump's move, which had been previewed to CNN by government sources and foreign diplomats, will give Congress 60 days to decide whether to reimpose sanctions lifted under the terms of the agreement.

While the administration is not expected to push Congress to go that far, since it would likely cause Iran to immediately walk away, proponents of the nuclear deal fear that Trump's decision will strike a severe blow at the deal's legitimacy.

A significant stiffened US policy toward Iran designed to tackle what the White House says are Tehran's destabilizing activities and support for terrorism could return the enemies to the cycle of confrontation and proxy wars of most of the last four decades, that could in itself cause the deal to slowly begin to unravel.

"If the President chooses to not certify, that already will be a negative step -- for one thing it will start a process of isolating us from our allies," Ernest Moniz, Obama's former energy secretary who helped negotiate the agreement, said on CNN's "New Day."

"If we went all the way and reimposed sanctions while Iran is in compliance ... this would be a slippery slope towards a bad outcome, something very much not in our national security interest," Moniz said.

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What are Trump's motivations?

The potentially grave consequences of Trump's decision, and the fact that the International Atomic Energy Agency, US allies and even the US government have said that Iran is in compliance with the agreement, have focused attention on Trump's motivations.

Critics say Trump is recklessly risking the deal, and thereby endangering US national security, simply to satisfy his fierce antipathy toward the agreement and to showcase a rare political win to his supporters.

Trump has twice previously been forced certify Iran's compliance, against his inclination and made clear he doesn't intend to do so again, even though Tehran is still honoring the pact.

The President is not alone in opposing certification of the deal. Some Republicans in Congress, including Arkansas Sen. Tom Cotton, and members of the conservative foreign policy establishment believe that his move on Friday will force America's European allies, China and Russia and eventually Iran back to the table to improve the deal.

The President has also complained that the 2015 deal does not allow UN inspectors access to military sites, an argument one foreign diplomat dismissed while wondering whether Trump understands what is in the pact.

"I'm not sure he's privy to all the details," the diplomat said.

Trump's supporters however argue that the deal puts the Iranians on a North Korea-style glide path to a nuclear weapon when it expires in 2025 -- a claim that proponents of the deal dispute. Those who back Obama's approach also slam the idea that there is a "better deal" to be had, as Trump has often said, as a myth or that other partners will agree to renegotiate.

"I don't know there is any guarantee that ever happens, there are just so many stakeholders here," said Brian Fleming, an official in the Obama Justice Department who worked extensively on the Iran deal and is now at the Miller & Chevalier law firm.

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Punting to Congress

The decertification by the President is only one aspect of the new Iran policy he will roll out on Friday.

Trump is also expected to unveil a toughened approach to respond to Iran's ballistic missile development, political maneuverings throughout the region and what the administration says is its support for terrorism, including for groups like Hezbollah and Houthi rebels in Yemen, officials have said.

By punting a decision on the ultimate destiny of the Iran deal to Congress, Trump can also try to personally avoid blame for the consequences that would follow if he formally killed the deal.

Once Trump has engineered the new policy direction, the deal's fate will be in limbo. Should Congress go ahead and decide to reimpose sanctions, it is all but certain that Iran would walk away. It could then likely reinstall centrifuges disengaged under deal and could race toward development of a nuclear device, a process that experts believe could take only a year or so.

Diplomats and sources who have spoken to CNN say they don't believe that even Republican hawks opposed to the deal want to destabilize it, and end up paying the political price for a potential march to war by the US.

Alternatively, lawmakers could decide to do nothing, effectively leaving the deal untouched.

In that case, Iran could decide that it is in its interest to remain in the agreement since it will still be reaping the economic benefits it gained via the lifting of sanctions.

Even so, it is uncertain whether this option would preserve the deal in the long term. Should European firms for instance reconsider investments in Iran under the shadow of potential future US sanctions, they could decide not to invest in Iran, and thereby lower the dividend that Tehran won by supporting the deal.

That could bolster hardline opponents of the deal inside Iran, as could the administration's desire to sanction individuals and entities in Iran's Revolutionary Guard Corps, which controls vast business interests in the country a state sponsor of terrorism.

"Longer term, this will be very humiliating and embarrassing for the Rouhani government," said Trita Parsi, author of the book "Losing an Enemy," Obama, Iran and the Triumph of Diplomacy." "They may be committed to the deal and they may not want to start messing with us, but their political strength will weaken and lead to a scenario in which they may lose power."