WASHINGTON — President Donald Trump long has railed against the Iran nuclear accord as “insane” and the “worst ever,” even though it has curbed Iran’s ability to develop or build a nuclear weapon since it went into effect in early 2016.

Despite strong support for the deal from all of America’s major allies except Israel, Trump has set a May 12 deadline to declare whether he will withdraw — or that the deal can be “fixed” to his liking. He has several options so the fallout either way is not yet clear.

Here are some things to know about the accord — and what is expected if Trump decides to pull out.

Q: Remind me: What is the Iran nuclear deal and what are Trump’s concerns?

A: In 2015, Iran negotiated an accord with six world powers — the United States, Britain, Russia, France, China and Germany. The deal lifted crippling economic sanctions on Iran in exchange for strict limits on its nuclear development program, which the international community feared Tehran could use to build a bomb.

The deal required Iran to disable most of the centrifuges it used to enrich uranium, to ship out most of its uranium stockpile, to ensure a heavy water reactor could not produce weapons-grade plutonium, and to submit to monitoring and verification by international inspectors. The International Atomic Energy Agency, the United Nations’ nuclear watchdog agency, has issued 11 reports confirming Iran’s compliance.

Critics say the deal gives too much leeway to Iran by setting time limits, called sunset clauses, on key restrictions. Tehran can install more centrifuges after 10 years, for example, and can resume research and development after eight years.

Trump has slammed the deal because it doesn’t also stop Iran’s production of ballistic missiles or its support for militant groups in the Middle East. The U.S. and its allies maintain separate sanctions on Iran for those issues.

Q: What is the May 12 deadline?

A: Congress requires the president to recertify every four months that Iran is complying with the deal, or it could reimpose sanctions. Trump has vowed not to sign another sanctions waiver, and May 12 is the next deadline for that.

If he doesn’t renew the waiver, Congress probably would restore U.S. sanctions on Iran’s Central Bank. The president would have to sign additional executive orders and take other legal steps to renew U.S. sanctions on about 400 other formerly blacklisted Iranian entities and individuals.

Some of that could happen within days.

Q: What would the impact be if the president declined to recertify the deal?

A: In a nutshell, it would make it more difficult for countries and companies to do business with Iran. Those who work through the Central Bank, especially importers of Iranian oil, risk violating U.S. sanctions.

If Trump reimposes all U.S. sanctions, many other Iranian banks and sectors, including the country’s critical petrochemical and automotive industries, would probably lose international partners.

Q: What would Iran do?

A: Here lies the most serious potential repercussion. If the United States pulls out completely, Tehran could declare the deal dead, blame Washington, and eventually resume its nuclear program.

Iranian Foreign Minister Javad Zarif posted a five-minute video on YouTube on Thursday to restate his country’s opposition to Trump’s position, saying Tehran would not “renegotiate or add on to a deal which we have already implemented in good faith.”

In New York last month, Zarif said that if the U.S. withdraws, Tehran would seriously consider “resuming at much greater speed our nuclear activities,” though he stopped short of threatening to produce weapons.

Iran also could respond by ending or limiting the inspections by the International Atomic Energy Agency of its nuclear facilities.

Q: What about the other countries that signed the deal?

A: All say they still support the agreement and will try to keep it alive if Trump pulls out. That could be tricky, since those governments and their companies would risk violating U.S. sanctions. One possibility would be for Trump to order “carve-outs” to exempt the other signatories, especially the Europeans, from repercussions.

French President Emmanuel Macron and German Chancellor Angela Merkel made back-to-back visits to the White House last month to lobby Trump in person. Macron told reporters there is “no Plan B” if the deal collapses. Russia has echoed the European warnings.

tracy.wilkinson@latimes.com