President Joe Biden's administration has been sounding more optimistic as it works to clear the final hurdles to revive the 2015 nuclear deal with Iran.
But before that can be fully achieved, the White House must first navigate the US Congress.
Frustration over the continuing talks in Vienna has come from both sides of the aisle in Washington, where questions around legislative approval of the deal persist.
A bipartisan group of 50 US congressman and women on Thursday expressed "deep concerns" over talks in Vienna, and demanded the administration consult with and provide Congress with the full text of any proposal to rejoin the Iran nuclear agreement.
"Strengthened with an estimated $1 trillion in sanctions relief over a decade, Iran and the IRGC [Islamic Revolutionary Guard Corps] would be an enormous danger to Americans at home and abroad, and to our allies," Congressional leaders wrote to Mr Biden.
"We must address the threat of a nuclear-armed Iran, stand strong against terrorists and protect American values and our allies."
Democratic Congressman Josh Gottheimer spearheaded the letter, which contained a total of 34 signatures from Mr Biden's own party and 16 Republicans.
A scenario where the Vienna negotiations produce a nuclear deal that Congress alone blocks is technically possible but “very unlikely”, said Jonathan Lord, a senior fellow and director of the Middle East Security Programme at the Centre for a New American Security.
But that does not mean a deal will not face Congressional hurdles and spark a potentially damaging backlash from US politicians in the run-up to November's midterm elections.
When the original nuclear deal, formally called the Joint Comprehensive Plan of Action or JCPOA, was approved in 2015, then-president Barack Obama signed into law the Iran Nuclear Agreement Review Act (Inara), which gave Congress oversight under a 30-day review period.
Whether or not the Inara is applicable to a deal from Vienna has been subject to debate, as it is not clear if a return to the JCPOA will be considered a new agreement.
Mr Lord, who previously worked as a staffer on the House Armed Services Committee, said the withdrawal from — and return to — a deal could be construed as an "amendment," or possibly a new deal, which the administration would likely submit to Congress under Inara.
Washington's lead negotiator, Rob Malley, testified to the Senate Foreign Relations Committee in May that any deal the administration reached would be submitted to Congress under Inara, initiating the 30-day review period that could lead to Congress passing a disapproval resolution.
Republicans have indicated in recent weeks that that could happen, over what they characterise as the Biden administration's inadequate engagement with Congress.
"It is completely unreasonable for this administration to think that a review could be favourable without a robust history of engagement with Congress, to include an increased tempo of briefings as negotiations reach their purported end game," Mike McCaul, the leading Republican on the House Foreign Affairs Committee, wrote in a letter last month.
Thursday's letter is not the first instance of members of Mr Biden's own party expressing similar sentiments.
Elaine Luria, a Democratic congresswoman, said in an April press conference: “The old JCPOA did not work and any new deal that does not prevent Iran from ever acquiring a nuclear weapon is unacceptable."
The Biden administration must walk a "tight rope" in keeping Congress informed of its policy objectives, without providing too many details that could allow opponents of the deal to scuttle negotiations, Mr Lord told The National.
"The administration could possibly build greater support or at least win less vocal opposition from the deal’s detractors if they shared with Congress its vision for a broader Iran policy and what steps it might take if a deal is reached," he said.
The legislative avenues Congress could pursue in altering a new deal on Iran could come in the form of Joint Resolution of Disapproval but the process would be difficult.
"Any Senator can introduce a JRD, but it would need 60 votes to pass. That’s a very tall but not insurmountable number," said Mr Lord.
In July, a bipartisan group of senators introduced legislation that would further enhance Congressional oversight on Iranian nuclear proliferation.
Democratic Senator Bob Menendez and Republican Senator Lindsey Graham introduced the Iran Nuclear Weapons Capability Monitoring Act of 2022, which would establish a joint task force led by the Department of State to monitor and regularly report to the appropriate congressional committees on Iran’s nuclear weapons and missile activities.
That goal would contradict conditions reiterated by Iran.
In a rare press conference on Monday, Iranian President Ebrahim Raisi issued a warning that any road map to restore a nuclear deal must require that international inspectors end their probe into man-made uranium particles found at undeclared sites in the country.
Still, Tehran has dropped a demand to delist the Islamic Revolutionary Guard Corps from the US list of foreign terrorist organisations.
