Obama can't deliver nuclear deal, US senators warn Iran
The letter, which is signed by the entire party leadership as well as presidential hopefuls Marco Rubio, Ted Cruz and Rand Paul, highlights that "anything that is not approved by Congress is a mere executive agreement".
The signatories warn both the Iranians and President Obama that Congress still wields significant influence over the ultimate fate of any deal. They "will remain in office well beyond" President Obama's time in the White House "perhaps by decades", they write.
"The coded message they are making with this letter is that sanctions relief will not be as far reaching as the Iranians think - unless they satisfy not only President Obama, but also Congress," said Timothy Stafford of the Royal United Services Institute.
Arming the hawks
The letter will give considerable ammunition to the hawks in Tehran who have always been sceptical about the US negotiating team's motives and its ability to deliver on promises of sanctions relief.
The last round of talks failed to secure anything concrete, and the two sides parted ways in November with nothing more substantial than an extension of the 2013 Joint Plan of Action.
Although it was a failure of sorts, it left the negotiation teams in the driving seat, albeit with their detractors back home emboldened and closing ranks around them.
"This gives tremendous fuel to the hardliners in Iran who are opposed to an agreement," said Kelsey Davenport, director for nonproliferation policy at the Arms Control Association.
"If the US can't honour its agreements then the Iranians have very little incentive to negotiate."
In Washington, President Obama and his team have also been fending off a concerted push for renewed sanctions.
The 2013 Nuclear Free Iran Act would ramp up petroleum and economic sanctions - but it has been kept at bay despite the backing of sixty senators, including seventeen Democrats.
In recent months, the tide has been turning in favour of hawks and the legislation is back on the table due to a concerted push by its chief proponents, Senator Bob Menendez (Democrat) and Senator Mark Kirk (Republican).
Advocates for stiffer sanctions argue that Iran only cooperates when its back is up against the wall, such as the decision to end hostilities with Iraq in 1988 when the country was completely exhausted, and the 2003 voluntary suspension of enrichment activity when fears of US designs for regime change within the "axis of evil" were at their apex.
Deal or no deal
Iran has, however, honoured its commitments in the interim deal - but if more sanctions are put on the table then there is a very real threat it could cause the complete collapse of the talks.
This is believed to be the intention of at least some of the letter's signatories. The plan is not so much to force a better deal but rather to ensure there is no deal at all.
Tom Cotton, the Republican senator who organised the letter of warning to the Iranian leaders, said in a speech at the far-right Heritage Foundation last week that the pending bills and push for greater Congressional involvement are intended to sabotage the negotiations.
"Certain voices call for congressional restraint, urging Congress not to act now lest Iran walk away from the negotiating table, undermining the fabled yet always absent moderates in Iran," he said.
"But, the end of these negotiations isn't an unintended consequence of Congressional action, it is very much an intended consequence."
The release of the senators' letter to Iran comes less than a week after Israel's Prime Minister Binyamin Netanyahu made a highly controversial address to the US Congress in which he feverishly railed against a nuclear deal with Iran.
He claimed the terms outlined by Barack Obama would "inevitably lead to a nuclear-armed Iran whose unbridled aggression will inevitably lead to war".
His bombastic and highly politicised speech became a highly partisan debacle and isolated many Democrats, many of whom either boycotted the session or were openly critical once it had finished.
"The Netanyahu speech seems to have changed some of the political considerations in Washington and some Republicans see it as a vote winning issue," explained Timothy Stafford, a research analyst with RUSI's proliferation and nuclear policy team.
Even though some Democrats support a tougher stance on Iran - and even if it means undermining President Obama's efforts in Vienna - this most recent letter was only signed by Republican senators.
The purely partisan backing to the letter suggests that the hardliners in the Republican party would rather go it alone without the Democrats on board.
Come election time they can then claim to be the ones who play hardball on foreign policy, remain unequivocal in their support of Israel and will not budge on Iran.
Taunting for failure
The talks continue, but if there is no meaningful agreement by the June 30 deadline, they could be fatally wounded.
They failed in November, but the two teams patched together a compromise to keep the process on track.
If they stumble again, it is likely the hawks in both Tehran and Washington will be so far vindicated that it will make the whole process untenable.
"At the moment you have people on both sides who want the talks to fail. They are almost taunting the other side to break the agreement somehow so they can say 'we told you so'," said Stafford.
President Obama will be loath to see his signature foreign policy collapse - and if American politicians are seen to be sabotaging negotiations it could seriously undermine their international standing.
In a January editorial in The Washington Post, the foreign ministers of the UK, France and Germany - together with the EU's high representative for foreign affairs - outlined their concerted opposition to the proposed bill for enhanced sanctions.
President Obama's room for manoeuvre is narrow indeed. "If we can't get a deal," he said last month, "as I’ve said to Congress, I'll be the first one to work with them to apply even stronger measures."
The Iranian Majlis has already prepared legislation that mandates a return to enrichment should the US adopt further sanctions.
Iranian and US opponents to a nuclear deal are lining up their ducks and the window of opportunity for President Obama and President Rouhani to stop them from shooting is rapidly closing.