Iranian President Hassan Rouhani's office struck an optimistic tone this week on reaching a potential agreement with the US that would bring both countries back into compliance with the nuclear deal.
Although the US State Department noted that negotiators have made some progress, a senior US official said that several differences between the two sides remain.
But even if President Joe Biden can clinch a deal with Iran that would have Tehran reinstate limits on its nuclear programme in exchange for US sanctions relief, such an agreement may have to survive a vote in Congress.
Republicans have made little secret of their intent to trigger a vote to block sanctions relief for Tehran in any new deal by using a 2015 law known as the Iran Nuclear Agreement Review Act.
Mike McCaul of Texas, the top Republican on the House foreign affairs committee, led 21 other members of his party in writing a letter to Secretary of State Antony Blinken this month arguing that any agreement would bring on a statutory review period in Congress.
This would allow Republicans to call for a vote that could kill the deal using expedited procedures laid out in the 2015 law.
“It is impossible to resume mutual compliance with the [Iran deal] as written and considered by Congress six years ago as though it were the continuation of the same agreement,” Mr McCaul and his allies wrote.
They listed Tehran's ongoing breaches of the accord.
Since former president Donald Trump left the deal in 2018, Iran has gradually ratcheted up breaches.
These include: enriching uranium at levels as high as 60 per cent purity; stockpiling enriched uranium at levels that are 16 times more than permissible under the accord; and installing advanced centrifuges at its nuclear sites.
“These violations make it impossible to simply ‘return’ to the [Iran deal], because Iran’s non-compliance has changed the deal itself,” wrote Mr McCaul.
He argued that Tehran's research and enrichment made the accord "irretrievably broken", meaning it "can no longer be complied with as drafted".
The non-partisan Congressional Research Service think tank offered a slightly different assessment – albeit one that could still lead to a vote in Congress.
A January report from the think tank said that the ability to trigger a vote under the statutory review period "may depend, among other things, on whether the agreement remains identical to the original or is substantively amended".
“If the United States were to renew the terms of the agreement without change, the president might reasonably conclude that, based on the plain meaning of the relevant statutory language, [the Iran Nuclear Agreement Review Act] would not require the transmittal of the written agreement again," the Congressional Research Service said.
"Other questions may arise if the parties were to amend the [Iran deal] prior to the US rejoining it.”
Throughout the six rounds of indirect talks in Vienna, the Iranians have demanded that the Biden administration lift the additional sanctions Mr Trump imposed on top of the sweeping sectoral sanctions that former president Barack Obama relieved under the original deal.
"I think what's holding up the talks in Vienna is the sanctions on the supreme leader's office," Kenneth Katzman, an Iran specialist at the Congressional Research Service, told The National.
The Trump administration imposed sanctions against supreme leader Ayatollah Ali Khamenei and his office, as well as president-elect Ebrahim Raisi and the outgoing foreign minister, Javad Zarif, in 2019.
If the Biden administration agrees to lift any of these sanctions as part of its deal to revive the nuclear accord, it could qualify as a substantive amendment, leading to the congressional review period that would allow Republicans to force a vote on killing the agreement in Congress.
Even then, Republicans would face an uphill battle in a Democrat-held Congress – though they could make life politically uncomfortable for some of Mr Biden’s high-profile Senate allies by putting them on record with regard to Iran sanctions relief.
Senate Majority Leader Chuck Schumer voted against the original deal in 2015, as did Bob Menendez of New Jersey, the chairman of the foreign relations committee.
Centrist Joe Manchin of West Virginia has used the 50-50 split this year in the Senate to flex his muscles and Ben Cardin of Maryland also voted against the original deal.
All four senators enjoy close ties with the powerful American Israel public affairs committee, which lobbied hard against the original deal.
The new Israeli government under Prime Minister Naftali Bennett has continued to lobby the Biden administration against nuclear deal re-entry.
Despite his initial vote against the deal, Mr Cardin did not shut the door on lining up behind any potential new agreement that Mr Biden may reach.
"I've been in touch with the administration," Mr Cardin told The National.
“They’re sharing information with us. I think their objective on what they’re trying to get from Iran are ones that I agree with, so let me see how they’ve worked it out.”
For his part, Mr Menendez joined Lindsey Graham of South Carolina in a recent Washington Post op-ed arguing for the creation of a nuclear fuel bank for Iran as part of a broader deal that would also address Tehran's ballistic missile programme and its support for proxies throughout the Middle East.
“Such a deal would have a better chance of garnering wider, bipartisan support in the United States, which would also send a stronger signal to Iran about its durability,” the senators wrote.
Asked about a potential vote in Congress, White House Press Secretary Jen Psaki told The National that "it's a little bit ahead of where we are".
“When we get to a point, if we get to the point where there’s a deal, we’re happy to have the discussion about the legislative vote count.”
But even if Mr Biden faces a few Democratic defectors on a potential Iran deal vote, Republicans will have a hard time mustering enough votes to kill any new agreement.
Democrats remain in firm control of the House of Representatives, and the procedural mechanism known as the filibuster – which has inhibited much of Mr Biden’s agenda so far – will provide him with a further buffer in the Senate.
The filibuster would require 60 votes for any resolution to block sanctions relief, meaning Senate Republicans would have to convince 10 Democrats to vote to kill a new deal – an unlikely prospect.