With the negotiations on a comprehensive nuclear deal reconvening next Tuesday in Vienna, a vocal minority of hardliners have turned up the heat on President Hassan Rouhani and his nuclear team, regarding them as too soft.
Last weekend, the president's opponents gathered at a meeting they called "We're Worried". They voiced their concerns that the government was surrendering its rights to the West out of desperation for a deal to secure extensive sanctions relief.
Its location was symbolic: the former US embassy in Tehran where 52 diplomats were taken hostage in 1979.
Suspecting Iran is developing nuclear weapons capability, Washington and its allies have imposed tough embargoes on the country, which have battered the national economy.
But hopes are growing that an interim nuclear agreement struck last November could lead to a permanent resolution.
"The whole nation believes the main intention of the United States is fully halting the Iranian nuclear programme," the state news agency, Irna, reported conservative member of parliament Fatemeh Alia as saying at the meeting.
"Since the beginning of drafting the final nuclear agreement with Iran is scheduled for 13 May this year, the analysts, the university students, the elites, and the university professors wish to express their worries to the nuclear negotiators," she added.
Her words are among a string of attacks against Mr Rouhani and his government who took office last August vowing to improve Iran's relations with the world, resolve the nuclear confrontation and breathe fresh life into Iran's economy.
Nine months on and Hassan Rouhani, a former nuclear negotiator, has made some inroads but faces increasingly desperate conservatives who saw a substantial erosion of their political base in the presidential election last June.
"Hardline opposition to Rouhani represents an extreme minority," said Sadeq Zibakalam, professor of political science at Tehran University.
"The total vote they captured in the election was four million, out of an eligible population of nearly 50 million. They're concerned about losing their grip on power, their economic positions and their political positions," he added.
Some of his detractors, however, are influential. They comprise hardline members of parliament, commanders of Iran's elite force, the Islamic Revolutionary Guard Corps (IRGC), conservative members of the clergy and a broader-based conservative group in the ruling elite.
Concerned about an undermining of the Islamic Republic's revolutionary principles and an unstoppable momentum towards radical change, they are refusing to back down.
A new low-budget film about the president, made by a company close to the IRGC, has been denounced by Mr Rouhani and his allies who see it as an attempt to tarnish his public image.
Critics have also repeatedly targeted Iranian Foreign Minister and chief nuclear negotiator Mohammad Javad Zarif. As the architect of Mr Rouhani's diplomatic outreach he is most openly exposed to attacks.
In their most recent campaign, Mr Zarif was summoned before parliament for a grilling over his comments on the Holocaust which he described as a "horrific tragedy" in an interview earlier this year.
Iranian conservatives have traditionally downplayed, or denied, the Holocaust, regarding it as an exaggeration by arch-foe Israel. Mr Zarif's remarks caused uproar but he survived a vote of no-confidence in the assembly.
Push for progress
Underpinning the government's progress is the support of Iran's most powerful figure, Supreme Leader Ayatollah Ali Khamenei, who has the final say on nuclear policy. Mr Khamenei has so far given his support to the talks but is hedging his bets in case the diplomatic initiatives collapse.
Aware of the pitfalls of Iran's multi-layered power structure and that the supreme leader's support may not endure for long, Mr Rouhani is pushing for rapid progress, say analysts.
He has repeatedly reminded the public of the leader's endorsement and castigated some for trying to "derail the government" in its attempts to win sanctions relief.
But his critics are not just riled by his foreign policy ambitions.
They have so far managed to impede the president's attempts to ease social and political restrictions in Iran, one of his election campaign promises.
Also, his attempts at reforming the economy jeopardise many who have grown wealthy on the country's skewed financial situation.
Only the influential and wealthy minority have been able to profit from the conditions that have come about through the embargoes, which have brought inward investment to a standstill and a dramatic rise in unemployment.
President Rouhani's strategy is divisive and a gamble.
"The hardliners are uncomfortable with Rouhani's broader agenda - political liberalisation at home and an economic programme that foresees a smaller role for the state and Iran's version of economic oligarchs in the economy," said Shaul Bakhash, professor of Middle East studies at George Mason University in Virginia.
"Clearly, at home, Rouhani has a difficult row to hoe. He's facing resistance in his attempt to curtail the vast role of the Revolutionary Guards in the economy and entrenched interests will oppose his attempts at economic rationalisation."
Marcus George is a former BBC and Reuters correspondent now based in the Gulf.