Beginning on January 20, the decades-long dispute over Iran's nuclear ambitions will enter a new phase. Tehran has reached agreement with the P5+1 - the permanent members of the UN Security Council plus Germany - on how to implement the interim deal reached in November 2013.

"The decision to start implementation of the November nuclear accord shows that diplomacy is gaining further momentum," Trita Parsi, president of the National Iranian American Council, told DW.

Iran will roll back part of its nuclear program as part of the six-month deal, agreeing to limit uranium enrichment and to open its nuclear program to daily inspections by experts from the International Atomic Energy Agency (IAEA). From that point, the clock then begins ticking on a six-month deadline for a final nuclear agreement.

In return, Tehran will be allowed to retrieve some $4.2 billion (3.1 billion euros) in seized oil revenue, Iranian Deputy Foreign Minister Abbas Aragchi told Iranian state television. However, these funds will be dispensed in installments that could be halted if Iran violates the accord.

Threat from Washington

US President Barack Obama in a statement hailed the breakthrough interim nuclear deal as the result of "unprecedented sanctions and tough diplomacy." He warned that broader sanctions would be "vigorously" enforced should Iran fail to meet its commitments.

But the biggest threat to the fragile progress appears to be coming from Capitol Hill. A growing coalition of Republican and Democratic lawmakers is pushing the "Nuclear Weapon Free Iran Act of 2013" to enact new economic sanctions on Iran. At least 59 senators currently back the package, arguing that their bill will ensure Tehran doesn't renege on its commitments or back out of the diplomacy.

"Being tough on Iran is very popular in the United States," Professor Gary Sick, an Iran scholar at Columbia University and a former staff member of the US National Security Council focusing on Iranian affairs, told DW. "They are saying: if some sanctions are good, then stricter sanctions are better."

Obama, however, has warned against the move. "Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation," he said.

Challenging a final deal

The momentum of senators supporting the Act has been slowing down in recent days. In particular, a growing number of Democrats are having second thoughts about supporting the sanctions legislation. However, Sick pointed out that the foreign policy battle represented a direct challenge between pro-Israel forces in Congress and the Obama administration. But the president, he said, was doing his utmost to take the wind out of their sails.

"The first stage of the deal goes into effect one week from now and the administration is anxious to make the specifics of the deal public," Sick said. "This is a very good deal and gives the Obama administration more credibility."

Parsi said President Obama should be commended for having shifted US policy away from sanctions and toward diplomacy. "To reach a final deal, however, even greater leadership is needed," he said. "The most valuable concessions from the Iranian side will only come when the US puts on the table the most valuable concession it can offer: the lifting of broad nuclear-related sanctions."

However, even if the Nuclear Weapon Free Iran Act does not make it onto the floor for a vote, the process has more far-reaching implications, said Parsi.

A final deal on lifting sanctions against Iran - with central bank and oil sanctions in question - needs the approval of Congress, he said. However, it will be difficult to start the necessary debate.

"It poses a very serious problem because it delays the ability of the administration to start negotiating a deal. But the sooner the conversation about lifting sanctions can begin in the US - rather than just delaying new sanctions - the sooner the US can push the process toward a final, comprehensive nuclear deal with Iran."

The foreign policy battle in Washington will not go unnoticed in Tehran. In Iran, hardliners have already called the deal a "poison chalice" and are threatening legislation to increase uranium enrichment. Deputy Foreign Minister Aragchi has highlighted its reversibility, saying that any new sanctions would halt the deal.