It was one of Donald Trump's signature campaign platforms.

At rally after rally across the country he described then-US president Barack Obama's Iran nuclear pact as "disastrous" and the "worst deal ever".

"This deal, if I win, will be a totally different deal. This will be a totally different deal," he said.

A May 12 deadline looms for him to either re-certify the arrangement as required every 90 days, or dump it, and while the latter would satisfy his supporters, he's under serious international pressure not to do it.

And now the President has brought the deadline forward, saying he will announce his decision within 24 hours.

So why does the President have such a problem with it?

The deal suspends a range of sanctions on Iran in return for limits on enrichment of uranium fuel, a freeze on nuclear weapons production, reducing the country's nuclear stockpile, and access for international inspectors.

While the International Atomic Energy Agency says Iran is complying, the President believes it's not doing so in the spirit of the agreement and fears that a 2025 sunset clause will allow weapons production to again quickly ramp up.

What if he pulls out?

Other countries who are party to the six-nation deal are pressing the President to work on fixing the problems within the arrangement, rather than dumping it altogether.

They argue that throwing the deal out now will accelerate an arms race in the Middle East.

"If they do get a nuclear weapon, you're going to get an arms race in the Middle East," UK Foreign Secretary Boris Johnson said on Fox and Friends.

"You're going to have the Saudis wanting one, the Egyptians, the Emirates. It's already a very, very dangerous state at the moment.

"We don't want to go down that route and as I say, there doesn't seem to me at the moment to be a viable military solution."

What has Iran said?

Iranian Foreign Ministry spokesman Bahram Ghasemi has foreshadowed dire consequences if the United States does pull out of the deal.

"Quitting the nuclear deal is not an easy task and Iran's reaction toward such an action will be an unimaginable one for America, and will be very painful, which will definitely make the United States regret it," he said.

"If America pulls out of the nuclear deal, they will have to pay a very heavy price and it will be a turning point for developing mistrust in the US administration further.

"Also, world opinion about the United States will dramatically change and will get worse."

Sorry, this video has expired Iranian President Hassan Rouhan says US would 'regret' pulling out of deal

However, Iranian President Hassan Rouhani seemed to indicate that a deal could still stand without the United States.

"What we want for the deal is that it's preserved and guaranteed by the non-Americans, and then the US pull-out will be OK," he said.

"We will get rid of the mischievous element that always caused trouble. And if what we want is not fulfilled, we have our own legal and lawful path and have our own plans."

What do we know about Trump's announcement?

White House officials suggested in recent days that the President's mind was "all but made up" to exit the deal.

However, it has also been hinted that a partial exit may be an option, although how that would work has not been articulated.

It's previously been suggested that new "trigger points" could be imposed to automatically re-impose sanctions on Iran if they're breached.

The aim would be to strengthen the enforcement of the deal and to target Iran's ballistic missile program.

The President is partly motivated by Iran's support for Syrian President Bashar Al-Assad and pressure from key US ally Israel and President Benjamin Netanyahu.

Iran denies aiming to develop nuclear weapons.

What else could happen?

The deal does include a dispute resolution mechanism.

It's possible that the President could exploit it by refusing to renew the sanction waivers while allowing new Secretary of State Mike Pompeo to continue negotiations with European nations.

"We are determined to make sure it [Iran] never possesses a nuclear weapon," Mr Pompeo said during his first visit to the Middle East as Secretary of State last week.

"The Iran deal in its current form does not provide that assurance."

If the deal collapses there are major implications for international banking, oil and aviation companies that re-entered the Iranian market after crippling economic sanctions were lifted.

Several companies have already slowed their expansion awaiting a decision from the Trump administration after the President disavowed the deal last year.