Washington (CNN)US lawmakers weren't sitting at the table with Iran during the nuclear negotiations in Switzerland. But they were certainly the elephant in the room.
Is Congress sabotaging a nuclear deal with Iran?
Now, with a new bill poised to pass that would give Capitol Hill a say on a future nuclear deal, the Republican-controlled Congress will have even greater power to determine the fate of the accord.
U.S. and Western diplomats involved in the nuclear negotiations already expected Iranian negotiators to drive a hard bargain, and they fear the new legislation could undermine faith in Obama's ability to implement an agreement. But while the diplomats aren't happy with the legislation, they are relieved it isn't worse.
"It's not brilliant, but it does feel like it gives us the space we need" to complete a final deal by the June 30 deadline, one Western diplomat said. "I would say we are lukewarm to it."
The new version of bill was the product of last-minute concessions by Senate Republicans that kept too many Democrats on board for the White House to be confident it could sustain a veto.
It gives Congress at least 30 days to review the deal, during which time President Barack Obama would be unable to waive any U.S. sanctions previously imposed by Congress.
The original bill tied the White House's hands more tightly, giving Congress a 60-day review period and requiring Obama to certify that Iran was not supporting terrorism against the United States.
The new measure requires the White House to submit a nuclear deal to Congress by July 10, which another Western diplomat said gives negotiators "the necessary time to finish the job."
"The good news is that nothing happens before July 10," the diplomat said. "After this, nobody knows."
The diplomats say they will have a better sense of how the legislation will affect the negotiations when talks resume on April 21 and the U.S. delegation will be forced to explain the effects of the congressional measure.
But an emboldened Congress will undoubtedly complicate efforts to reach a deal. Republicans have been highly critical of the framework agreement worked out earlier this month, charging that it won't keep Iran from getting a nuclear bomb, and some Democrats have also expressed skepticism. And legislators on both sides of the aisle have been displeased at not having a greater role in the process, something the new bill seeks to rectify.
For now, Iran is playing it cool about the legislation. President Hassan Rouhani called it a U.S. domestic issue that should not impact the talks with the P5+1 negotiating team, comprised of the U.S., U.K., France, Germany, Russia and China.
"We are in talks with the major powers and not with the Congress," Rouhani said this week. "What the U.S. Senate, Congress and others say is not our problem."
But Rouhani only needs to look at his own parliament to know that lawmakers love to play the spoiler. This week the nuclear committee in the Iranian Majlis released a factsheet outlining revisions that would need to be made on the framework in order for a final deal to be reached.
But the document the Iranians released reads like Tehran's position months before the framework was announced.
The preliminary deal, reached in Lausanne, Switzerland, on April 2, included what negotiators called "parameters" for an eventual nuclear deal. A final pact with all technical details resolved is to be reached by the end of June.
The framework calls for Iran to keep 10,000 of its 19,000 centrifuges to enrich uranium, while the Lausanne framework left the Iranians with about 5,000. It also slashes the duration of the deal to five years from the 10 years agreed to in Lausanne.
Diplomats involved in the talks seemed unfazed. In many ways, the document isn't worth the paper it's written on. While hardline opposition doesn't necessarily help Iran's rulers seal a deal, the Iranian parliament has considerably less power than Congress to impact a deal -- and perhaps none.
Furthermore Rouhani is in a weak position to use the parliament as leverage, should he want to, after dismissing Congress as insignificant.
But the criticism proves there is at least as much opposition to the deal among hardliners in Tehran as there is among lawmakers in Congress.
What is more troubling for the chances of a deal are comments over the last week by both Rouhani and Supreme Leader Ayatollah Ali Khamenei -- the real power broker -- that a deal would not be possible unless sanctions were lifted immediately once an accord is signed.
When the framework was announced in Switzerland, U.S. officials asserted that the lifting of sanctions would be phased out gradually as Iran complied with the final deal's core nuclear restrictions.
It was predictable that Iran, having refused to sign the framework as a formal document, would have a different take.
After the United States released its fact sheet on the framework agreement, Iranian Foreign Minister Javad Zarif quickly dismissed it as "spin" and fired off a series of tweets that sanctions would be lifted immediately.
"Iran/5+1 Statement: 'US will cease the application of ALL nuclear-related secondary economic and financial sanctions.' Is this gradual," Zarif tweeted.
That was followed by "Iran/P5+1 Statement: 'The EU will TERMINATE the implementation of ALL nuclear-related economic and financial sanctions'. How about this?"
Western diplomats said that despite Tehran's push-back, Iranian negotiators left Lausanne knowing full well the myriad of U.S., E.U. and U.N. sanctions would be lifted according to various mechanisms depending both on the types of sanctions and actions Iran would first have to take.
"We always said that all sanctions would not be lifted on day one," a western diplomat said. "There is no way to make that happen even if we wanted to."
For example, sanctions imposed by the U.N. Security Council related to proliferation can only be lifted once the U.N. nuclear watchdog, the International Atomic Energy Agency, certifies that Iran has made specific progress.
"They always wanted everything lifted on day one. But they know that it will not be the case," another Western diplomat said. "They will be incremental, with a snap-back provision if they violate. Otherwise there is no deal."
Both Secretary of State John Kerry and Energy Secretary Ernest Moniz have played down these discrepancies as differences in what each side has chosen to emphasize as opposed to actual content.
But these so-called dueling narratives appear to be more about differing interpretations of what was actually agreed to in the Lausanne framework.
It is far from clear what sanctions relief would come once Iran signs on the dotted line at the end of June and which sanctions will be lifted once Iran complies with its obligations.
Moreover, the White House now acknowledges the actual details of the timing of the phase-out of sanctions and a "snap-back," allowing Obama to reapply the measures if Iran violates the deal, have not yet been agreed to.
"Part of this is that there is some spin going on by the Iranians, and part of it is that there are details that still have not been agreed to and need to be worked out," another Western diplomat said.
All of this strongly suggests that reaching a final deal will be difficult, even without the additional complications posed by Congress' new role.
While the devil will be in negotiating the details of the deal, diplomats believe the fact that the Iranians agreed to the outline suggest Khamanei, the primary power center in Iran, signed off on the terms.
"We feel we crossed over the line where the political decisions were made," a Western diplomat said. "There was a moment where we said we couldn't go further if Iran didn't make those decisions. But they did and that was the biggest progress."
But the diplomat quickly added, "That doesn't mean there are not difficult discussion and negotiations ahead."