For years, US lawmakers have carefully crafted legislation aimed at reining in Iran's suspect nuclear program and moves to provide even some sanctions relief are likely to be met with suspicion on Capitol Hill.
Even as the nations leading the talks with Tehran -- the United States, Britain, France, Russia, China and Germany -- met for the first time with the new Iranian leadership in Geneva last week, US lawmakers issued warnings to the negotiators to be on their guard.
Details of what Iranian officials proposed during an hour-long power point presentation in English behind closed doors remain confidential, but the talks were hailed as "serious and substantive" and the broad contours are not hard to discern.
The six powers want the Islamic republic to halt enrichment of uranium to 20 percent, which brings it to weapons grade, and dismantle its weapons-related infrastructure, in return for relief from crippling economic sanctions imposed by the US, UN and European powers.
But enmity and suspicion between Iran and the US stretch back decades to the Islamic revolution and the 444-day siege of the US embassy in Tehran in 1980. And any deal could be a hard sell to hardliners in both nations.
US chief negotiator, Undersecretary of State Wendy Sherman, has already been debriefing lawmakers by phone and is set to testify at a series of classified briefings before she returns to Geneva for the next talks on November 7 and 8.
"I don't think the issue's convincing the American people, I think it's convincing the people in Congress who want to pass sanctions no matter what," said Alireza Nader, senior policy analyst at the RAND Corporation think tank.
Indeed, draft legislation for new US sanctions targeting the automobile sector and Iran's foreign reserves is awaiting a vote in the Senate, after being adopted in July by the House of Representatives.
President Barack Obama has some authority to ease some US sanctions hitting the petroleum and banking sectors, mostly those slapped on third countries doing business with Iran.
And the New York Times reported last week the White House may be considering unblocking some $10 million in frozen Iranian assets in the United States to give the Rouhani leadership some access to badly needed foreign exchange reserves.
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But Joel Rubin, director of policy at the Ploughshares Fund, a non-government organization working for a nuclear-arms free world, warned that if Congress continues to press sanctions it may become "a spoiler" to the delicate diplomatic maneuvering under way.
"Congress has done a very good service to help create a pressure component to the overall strategy on Iran," Rubin told AFP.
"The danger is Congress will continue to do what it does in promoting additional measures, that would actually complicate the environment."
He argued that while the world's toughest sanctions regime had brought the new Iranian leadership of President Hassan Rouhani to the table, it had failed to halt Iran's nuclear drive, which the West suspects masks a bid for a nuclear bomb.
Iran is now believed to have something between 18,000 to 19,000 centrifuges, key to the enrichment operation, while its new heavy water reactor at Arak may come online in 2014.
"Demands for Iran to completely dismantle it's nuclear program are very unreasonable and could sabotage negotiations," Nader told AFP, referring to some of the louder voices in Congress as well as allies such as Israel.
"The Iranian nuclear program is a reality. They have a relatively big infrastructure, they've mastered the technology.
"I think at this point we have to make sure that they don't move towards a weapons capability, rather than getting rid of the program completely."
Daryl Kimball, executive director for the non-partisan Arms Control Association, agreed, saying that "from a non-proliferation standpoint" zero enrichment would be the ideal, but it is "clearly a non-starter."
"If the P5+1 were to insist upon zero enrichment and the dismantling of its core facilities, it's more than likely that the Iranians will simply just walk away," Kimball told AFP.
He argued the choice was now "between limiting Iran's nuclear program and having tougher IAEA safeguards... or having Iran continue to improve its program, unconstrained uranium enrichment and the increasing likelihood that there's a military conflict over Iran's nuclear program."
"This maybe our last best opportunity to secure an agreement that bars against a nuclear armed Iran. If we lose this opportunity, both sides will lose out," he added.