Fierce criticism of the Iran nuclear agreement by Republicans seeking the US presidency has raised a big question in Tehran -- will future American leaders keep their side of the bargain?

Despite tension and continuing mutual mistrust, Iran's government and President Barack Obama's White House are partners in the same fight, telling their domestic audiences that the July 14 deal is as good as it gets.

But with the US presidential election only 15 months away, opponents of last month's historic pact -- particularly those who are lining up to replace Obama -- pour scorn on it.

No leading Republican contender has pledged to stand by the agreement between Iran, the United States and five other world powers. Several have promised to rip it up if they are elected.

On Wednesday, Republican presidential hopeful Jeb Bush called the deal a "farce", saying rules for inspecting Iran's nuclear sites were unclear.

Every such intervention raises doubt in Iran about whether the US "can follow through", said Foad Izadi, a partly US-educated political analyst and professor at the University of Tehran.

"If we get some crazy person in the White House the potential is there for the deal to fall apart," he said.

"It's not the next 18 months that bothers me but what happens when Obama goes. People are concerned about that."

When the nuclear deal's terms were concluded -- international sanctions on Iran will be lifted in exchange for curbs on its nuclear programme -- it came against heavy odds and opposition, particularly from Israel.

Republican presidential candidates and lawmakers have since kept the fight against the deal alive.

- Obama veto likely -

Iran and the US, the dominant international player in the talks led by Secretary of State John Kerry, broke off diplomatic relations in 1980, after the Islamic revolution in Tehran the previous year.

For many Americans, including representatives in Congress, the rancour with Iran is epitomised by the 444-day US hostage crisis. After students stormed the walls of the US embassy in Tehran they eventually paraded captive diplomats in black blindfolds.

The Republican-dominated Senate and House of Representatives are expected in September to pass a resolution opposing the nuclear deal.

Although Obama is likely to have enough votes from Democrats to veto that measure, the partisanship highlights the trouble the agreement could face should a Republican win the White House next November.

Amir Mohebbian, a political analyst and strategist with close ties to Iran's leadership, said that though they probably do not realise it, the Republicans are gifting Iran a "get-out" over the nuclear deal.

"If Mr Obama cannot manage this conflict between Republicans and Democrats we can say to the world 'we did everything we could', but the reason for this deal's defeat would not be Iran.

"No one could say Iran showed no flexibility and did not want to solve the problem. It would be seen as the fault of the United States."

Such a stance would also make other options regarding Iran's nuclear programme, including military strikes, much harder, he said.

"After these talks, any hard multilateral action against Iran would be very difficult for the US to justify to European countries, especially after Iraq.

"If the US goes down the unilateral route it will be worse for them than for us," Mohebbian said.

- 'Irrational enemy' -

The battle in Congress over the deal has coincided with a public relations push in Washington and Tehran.

Iranian Foreign Minister Mohammad Javad Zarif, Tehran's lead negotiator in the nuclear talks, recently conducted several high-level briefings in the capital about the deal.

Despite the opposition in Congress, and criticism from some Iranian generals, Zarif said on July 29 he had "no concern or worry" that the agreement's terms would be fully implemented within months.

Even so, the Islamic republic's supreme leader Ayatollah Ali Khamenei has not conclusively backed the deal.

The 76-year-old has praised Zarif and his negotiators but repeatedly casts doubt on the trustworthiness of the United States.

"They think that through this agreement -- the fate of which is not clear as no one knows if it will be approved here or in America -- they could find a way to intrude into the country," Khamenei said on Monday.

As such the White House race stands to undermine America's position on the nuclear agreement, Mohebbian argued, particularly as no Republican candidate has presented a clear alternative.

"The US is showing no sign its political leadership is as powerful as it claims to be," he said, citing the Congress-White House split. "Our enemy is not behaving rationally," he added.