History thus suggests that external economic pressure matters, but the balance of domestic political forces in Iran matters at least as much—and it is the interaction between the two that matters most of all. The Islamic Republic's authoritarian political system is not nearly as static or monolithic as many casual observers assume. Rather, it is an arena for contestation between competing political actors and interests—and the winners of these battles can have considerable influence over the ultimate course Iran takes. To be sure, Supreme Leader Khamenei is the most powerful actor in the Iranian government, and he is the ultimate decider on the nuclear issue. But he is not omnipotent or unmovable. More often than not, Khamenei stays above the political fray, waiting to weigh in on controversial decisions until he has assessed the domestic power balance and the direction the political winds are blowing.

Iran’s domestic politics matter because competing factions place different values on the nuclear program relative to other national priorities, and they have fundamentally divergent diplomatic and economic worldviews. Iranian moderates—including both pragmatic conservatives and reformers—believe Iran’s national interests are best served by international recognition and integration. They value the country’s nuclear program, but they also worry that pursuing nuclear weapons could ultimately leave Iran less secure by worsening regional tensions and, by making Iran the target of sanctions, ruining the nation’s economy. Consequently, they may be willing to settle for a nuclear outcome in which Iran maintains some distant, latent capability to develop nuclear weapons under significant international constraints. Such a capability, in their view, would be sufficient to deter foreign adversaries if security conditions deteriorate, but would not put Iran so close to an actual bomb that it results in international isolation. For pragmatists like Rouhani, that latent status was achieved once Iran mastered uranium-enrichment technology, and they seem willing to trade away more advanced nuclear capabilities to achieve their higher-order objectives of sanctions relief and reintegration into the international community.

In contrast, Iranian hardliners—including so-called Principlists and traditional clerical conservatives—do not seek integration with the wider world. They embrace a narrative that portrays the United States, Israel and the West as unrelenting enemies hellbent on toppling the Islamic Republic and depriving Iran of the economic and scientific wherewithal to take its rightful place among the world’s great nations. They see resistance to the West as the core of Iran’s national identity. And they view economic self-reliance and the acquisition of a one-turn-of-the-screwdriver-away “threshold” nuclear capability or actual nuclear weapons as the only means of deterring Western aggression and realizing Iran’s regional ambitions. For this group, international threats and sanctions simply vindicate their worldview, encouraging them to escalate their own provocative counter-reactions.

In this clash of perspectives, Khamenei appears closer to the hardliners’ camp. But Khamenei is also concerned about the legitimacy and survival of the system as a whole, which was badly damaged by the rigged 2009 elections and the mishandling of foreign and economic policy during Ahmadinejad’s tenure. Rouhani's sweeping election victory thus mattered not only because of the new president’s own preferences, but because the election itself signaled to Khamemei that some policy shift was required in order to maintain domestic legitimacy. Anxious to shore up the system, Khamenei appears willing to give Rouhani a chance to resolve the nuclear impasse, but only so long as the president and his negotiating team do not cross the leader’s red lines, especially as it relates to defending Iran’s asserted right to enrichment.

If Rouhani can maintain sufficient elite consensus, Khamenei may ultimately agree to meaningfully roll back Iran’s program as an act of “heroic flexibility” to relieve the economic pressure created by sanctions. But he will not support total capitulation. Given the significant financial investment—estimated to be at least $100 billion—and political capital the regime has expended to master uranium enrichment, the supreme leader will not agree to completely dismantle Iran’s program as many in Congress demand. Indeed, Khamenei probably fears such a humiliation more than he fears economic collapse or targeted military strikes against his nuclear facilities. If Khamenei senses Rouhani and Zarif are headed in that direction, he will likely pull the rug out from under continued negotiations, regardless of U.S. threats to escalate the pressure further. And cognizant of this fact, Iranian hardliners will seize on any sign that Rouhani is being suckered by the West to try to sway the leader's decision.

What does all this mean for the current debate in the Senate over new Iran sanctions? It means that any member of Congress truly committed to a diplomatic outcome should recognize America’s acute interest to ensuring that Iranian moderates maintain their fragile momentum within Iran’s political system. The Revolutionary Guard and other hardliners are already fighting a rearguard action against the Geneva agreement, with a war of words breaking out in recent weeks between Zarif and the Guards’ top commander, Major General Mohammad Jafari, over the course of Iran’s nuclear and foreign policy. These same forces would undoubtedly seize on Congressional legislation threatening new sanctions and demanding de facto nuclear surrender as the latest example of American perfidy, using it to rebut Rouhani’s claim that an accommodation with the West that protects core Iranian interests is possible. Hardliners have consistently argued that Iranian compromise is just a prelude to greater U.S. pressure. Khamenei suspects this too. Threatening new sanctions in the immediate aftermath of the first meaningful Iranian concessions in a decade, as the proposed Senate legislation does, risks validating that view.

The Senate bill could also lead to provocative Iranian counter-reactions at an extraordinarily delicate moment for diplomacy. Indeed, nearly one hundred hardline Iranian parliamentarians have already drafted legislation that would mandate escalating enrichment to the nearly-bomb-grade 60 percent level if more U.S. sanctions are imposed. Given thirty-five years of distrust between Tehran and Washington, it would not take much perceived bad faith by either party to reverse the modicum of confidence built at Geneva. It is difficult to imagine negotiations surviving such a tit-for-tat retaliatory cycle.

Finally, Rouhani’s ability to forge elite consensus for the additional concessions required for a final nuclear deal hinges on his ability to deliver meaningful sanctions relief, not just avoid an increase in sanctions. Yet by imposing demands that Iran completely dismantle its enrichment program—which Khamenei, hardliners and the majority of the Iranian public view as unacceptable capitulation—prior to lifting U.S. sanctions, the proposed Senate legislation will make it extremely difficult for Rouhani to build a coalition in favor of further compromise. The net effect will be to make a comprehensive, peaceful resolution to the Iranian nuclear crisis more difficult to achieve.

In 2005, the last time Iran and the West had an opportunity for a nuclear breakthrough, Iran walked away from negotiations on a comprehensive accord because moderates were discredited. Hardliners came to dominate the Iranian political scene and the nuclear threat grew. History is not doomed to repeat itself, but it easily could if Congress inadvertently helps the forces of confrontation regain lost ground.

Colin H. Kahl is an associate professor in Georgetown University’s Edmund A. Walsh School of Foreign Service and a senior fellow and director of the Middle East Security Program at the Center for a New American Security. From 2009 to 2011, he was the Deputy Assistant Secretary of Defense for the Middle East.