If Donald Trump decides this week to withdraw his endorsement of the 2015 Iran nuclear deal, its fate and the potential for a major conflict will be determined by a complex battle in Congress.

No one is able to predict whether that struggle will lead to a reimposition of US sanctions, the collapse of the agreement and the rapid scaling-up of Iran’s nuclear programme. It could result in a compromise that leaves the deal alive but opens the way for a more combative policy towards Tehran on other fronts.

“We are on a tightrope. We don’t know what will happen,” a western diplomat said.

The congressional contest will pit most Republicans against almost all Democrats, hawks against doves, and will be played out under rules drawn up for a completely different set of circumstances.

The Iran Nuclear Agreement Review Act (INARA) of 2015 was designed for a situation in which Iran was breaking the international agreement signed in July of that year, and a US administration was trying to cover up Tehran violations as a means of preserving the accord.

The actual situation is one in which Iran is agreed by all signatories, including the US, to be abiding by its obligations, but the US president appears determined to kill off the deal regardless. To add another level of complication, neither the Republican majority nor the president wants to be seen as the assassin that inflicts the death blow.

Trump says he has made his decision and an announcement is expected on Thursday or Friday. Most signs are that he will not certify the deal, the Joint Comprehensive Programme of Action (JCPOA).

The president has changed his mind at the eleventh hour before and the UK has not given up. In a telephone call with Trump on Tuesday, British prime minister Theresa May “reaffirmed the UK’s strong commitment to the deal alongside our European partners, saying it was vitally important for regional security”, Downing Street said.

The British foreign secretary Boris Johnson insisted the UK regarded the deal as an historic achievement that had made the world a safer place. He reiterated this in calls with the US secretary of state Rex Tillerson and the Iranian foreign minister Javad Zarif.

If such last-ditch appeals fall flat and Trump washes his hands of an agreement he has called “the worst deal ever negotiated”, INARA gives Congress 60 days to decide whether to reimpose sanctions.

The vote can be passed in the Senate by 51 votes. The Republicans have 52 seats.

The Republicans have historically been…