• Since sanctions were lifted, $100bn in frozen Iran assets have been released

• Agreement was signed by Iran and six world powers in July 2015

• US president has until October 15 to certify or withdraw from the Iran nuclear deal

It was 10 days before the May 2017 presidential elections in Iran. A fired-up crowd of men in green shirts and women in purple headscarves packed a sports stadium in Tehran from floor to ceiling.

Not far from the former US embassy in the capital, they were waiting for the arrival of President Hassan Rouhani, who was campaigning for reelection.

Outside, thousands more were pushing and shoving, jockeying to get in.

As droves of people in purple armbands streamed into the Shiroudi Stadium, the afternoon heat continued to rise. Swirling exhaust fans were no match for the stifling humidity. Rouhani's campaign organisers started to distribute bottles of water, while supporters were frantically fanning themselves.

When Rouhani finally emerged on stage, the crowd roared his name, overpowering the thump of his campaign song. His voice was strained from campaigning.

Rouhani spoke of the country's aspiration for "freedom and reform". At times, he would stop to wipe off the sweat from his face.

Then came the repeated rallying cry of his supporters, even interrupting the president mid-speech. "Ma sabze, sabze, sabzim. Batoom banafshemoon kard."

"We are still green, but your batons have turned us purple," they said in unison, referring to Rouhani's purple campaign colour, the green colour that had dominated the reform movement during the 2009 election, and the police crackdown that followed.

The chant was a clear sign which side the reformers were on, despite their misgivings towards Rouhani during his first term in office.

Rouhani would ride on that momentum to claim a decisive re-election victory, and then push for social, economic and political reforms.

But, fewer than five months after victory celebrations, Rouhani's supporters are now worried that reforms could stall if US President Donald Trump makes good on his promise to tear up the Iran nuclear deal.

Iranian political observers have warned that Trump's threat could demoralise reformists and embolden the hardliners, sending the country back to a period of political uncertainty and economic recession, or worse, lead it to a military confrontation.

Political and economic anxiety

Saeid Golkar, an Iran expert at the University of Tennessee in the US and Chicago Council on Global Affairs analyst, told Al Jazeera that tensions and the threat of conflict help Iranian hardliners to "silence the moderates and consolidate their power".

Saeed Jalili, a Tehran-based journalist, agreed, saying hardliners "feed on the hostility with the West" to consolidate their influence.

While both doubted the possibility of armed conflict in the event Trump scuttles the deal, they said the US threat is already causing anxiety among many Iranians.

"People are worried of the possibility of the return of the sanctions and economic hardships," Golkar said.

Jalili added that businessmen, as well as the middle class and educated young people, are concerned that economic progress following the lifting of sanctions would be short-lived should the US and Iran return to political hostilities.

Since the Iran nuclear deal was signed in July 2015 between Iran and six world powers, Tehran has scaled back its uranium enrichment programme.

In exchange, sanctions on Iran's economy were lifted, and $100bn of frozen assets were released. Iran was also allowed to trade its oil and gas in the world market, allowing it to earn over $41bn in the fiscal year that ended in March 2017.

During the same period, tourism has also increased, hitting six million visitors, with an estimated revenue of $8bn.

Despite some progress, the US has still prevented American banks from dealing with Iranian businesses, and prevented US credit card companies from operating in Iran - actions Tehran said violate the deal.

US companies, including oil and gas firms, are also prohibited from doing business there, and those who have managed - such as the aircraft maker Boeing - are being subjected to strict regulations.

Trump factor

When Trump took office in January, relations between Washington and Tehran took a turn for the worse after a couple of years of detente.

Trump, who called the Iran nuclear agreement "the worst deal ever" during his campaign, has repeatedly vowed to end it since becoming president.

In September, during his first address at the United Nations, he called the Iranian government "a corrupt dictatorship" that exports "violence, bloodshed and chaos".

Since then, he has declared that Iran had "not lived up to the spirit" of the deal, hinting that his administration could withdraw from the landmark pact.

The deadline for Trump to make that decision is October 15. But he could make an announcement as early as October 12.

Angered by Trump's threats, the Supreme Leader Ayatollah Ali Khamenei declared Iran would not give in to US "bullying".

In his address before the UN, Rouhani criticised Trump's remarks as "ignorant, absurd and hateful rhetoric filled with ridiculously baseless allegations".

Golkar said that Trump's rhetoric alienates progressive and young Iranians, making it difficult for reformist leaders to open a room for compromise with the US.

'Dangerous ground'

For Ariane Tabatabai, an Iran expert and professor at Georgetown University, it is puzzling that Trump is escalating tensions with Iran as the US faces a "real crisis" with North Korea.

While the Iran nuclear deal is not perfect, she said it was "precisely designed to remove the urgency" of Iran advancing its nuclear energy programme.

"If we're looking at US foreign policy priorities, Iran should not be this high up given its actual capabilities," Tabatabai said.

While unconvinced the US wants war with Iran, she said the Trump administration has no viable alternative if it decides to abandon the nuclear deal.

"The situation we are in is incredibly dangerous," Tabatabai said.

"You have a situation where the two sides are back to not communicating, and their militaries and allies operate in close proximity to one another in a number of theatres. All this is a recipe for misperception and miscalculation."

Trump eyes designating Revolutionary Guard as 'terrorists'

Meanwhile, Golkar warned that apart from the possible termination of the nuclear deal, a more worrying prospect is Trump's threat to declare Iran's Islamic Revolutionary Guard Corps a "terrorist" organisation.

On Tuesday, a Revolutionary Guard spokesman had already warned that Iran would teach the US "a new lesson" if Trump goes ahead with that decision.

"If the US labels the Guard as a terrorist group, and decertifies the nuclear deal, Iran can react more harshly," Golkar said reforming to the special military unit loyal to the Supreme Leader.

"They will see this as a full plan for regime change in Iran," he said.

"Labelling the Guard can change the game completely. It has many unseen consequences, which can lead to a military confrontation with Iran."