In the basement of Washington’s swank Mandarin Oriental Hotel on a balmy spring day, the conference guests were finishing up their boxed lunches as the conversation shifted to their host’s pet topic — Iran. The Foundation for Defense of Democracies, perhaps DC’s premier neoconservative think tank, had gathered donors, supporters, press, and other interested parties for a two-day meeting on Middle East policy. And some of the Hill’s most rapacious hawks for sanctions on Iran were in the room that day to receive awards.

The moderator, a veteran Bloomberg reporter, hailed FDD executive director Mark Dubowitz as “the architect of many of the sanctions we have against Iran right now, who advised Congress on how to draft that legislation and has also advised Treasury and the White House on his opinions about sanctions.” The praise was telling. Although Dubowitz tried to give credit to Congress, the White House and the departments of Treasury and State, groups like the FDD play an outsize role in shaping policy on the delicate and potentially explosive issue of Iran’s nuclear program.

Since the moderate Hassan Rouhani was elected president of Iran last June, the Obama administration has engaged in an intensive round of diplomacy aimed at placing permanent curbs on that program. The talks have progressed further than anyone expected, with an interim deal in November that set a late July deadline for reaching a final accord. On Capitol Hill, though, diplomacy has been dismissed by a parade of influential naysaying hawks. And these organizations are already talking up ways of making sure that a deal, if one is reached, is dead on arrival.

Within Washington’s corridors of power, the institution that has done the most to focus attention on the alleged Iranian nuclear threat — Congress — has also been among the most skeptical when it comes to using diplomacy to do anything about it. But the members of Congress don’t come up with these ideas on their own. A handful of organizations — especially the FDD, the American Israel Public Affairs Committee (AIPAC) and the American Enterprise Institute (AEI) — do most of the legwork in shaping policy. An even smaller network of right-wing donors funds these groups (see the sidebar for more).

Over the past decade, this small network of advocates has become incredibly effective at getting its way. A 2010 bill slapping sanctions on foreign banks and companies doing business — especially oil business — with Iran passed the Senate 99–0, and a 2011 amendment sanctioning international companies dealing with Iran’s Central Bank passed 100–0. In 2012, another sanctions amendment passed the Senate 94–0, and a 2013 resolution backing Israel should it attack Iran was passed 99–0. “By far and away the most powerful voices are what you can term the hawkish groups on Iran policy,” says a former congressional aide.

In the boxing ring that is Washington, the match-up isn’t even. Compare, for example, the budgets of groups that oppose diplomacy with those that support it. Four of Washington’s pro-diplomacy groups are significant players on the Hill: the Center for a New American Security, the National Iranian American Council, the American Iranian Council and the Council on American-Islamic Relations. According to their most recent tax filings, these organizations boasted an annual combined budget of approximately $9.4 million.

Meanwhile, the latest tax filings for just two of the groups that push hardline policies, the FDD and AIPAC, have a combined budget of approximately $75 million. And that doesn’t include the annual budget of an AIPAC offshoot, the Washington Institute for Near East Policy ($8.7 million), or aggressive right-wing PR groups like United Against Nuclear Iran ($1.6 million), whose spokespeople are regularly quoted by national media.

All that cash helps produce papers and reports advising Congress, flashy DC conferences and other ways of accessing power. For example, a more modestly funded dovish group might request a meeting with members of Congress, but some members will meet only with advocates who bring along a constituent, which could require buying a plane ticket. “That’s obviously easier for lobby groups that have a lot of money, because they can fly someone out,” says Kate Gould, a lobbyist with the pro-diplomacy Friends Committee on National Legislation.

The hawkish groups skillfully work the Hill with regular briefings and frequent contacts with staffers. Their battalions of policy analysts and lawyers “package [sanctions] bills and hand them to congressional offices,” says the former Hill aide. They also assiduously ply the mainstream media, regularly providing op-eds and quotes in news coverage. In other words, this is a full-scale operation: the hawks generate the ideas, translate them into policy, shepherd bills through Congress, and celebrate their passage.

To see how deeply these groups have influenced Congress, one need only glance at the docket of House and Senate committee hearings on Iran. It’s at these hearings that members of Congress vie to burnish their credentials as being tough on Iran, calling for ever-harsher sanctions. “Congressional hearings are not weighted to be some objective analysis of some foreign policy issue,” says the former congressional aide. “The people who are calling the hearings have an agenda.”

Since November 2012, eleven separate hearings on Iran policy have considered a total of thirty-six expert testimonies from outside groups. Of that number, two neoconservative organizations dominated: FDD fellows made five appearances, and those from the AEI had four. Neoconservative allies like David Albright, who co-chairs a nonproliferation group with Dubowitz and spoke before Congress four times in this period, also gave testimony. All told, people associated with groups taking a hard line on Iran sanctions accounted for twenty-two of the thirty-six testimonies solicited by House and Senate committees.

Centrist think tanks, on the other hand, were underrepresented. Employees of the Council on Foreign Relations testified twice, while the Brookings Institution, the RAND Corporation, the Carnegie Endowment for International Peace and the Center for Strategic and International Studies fielded only one witness apiece over the period reviewed by The Nation. Experts from dovish think tanks hardly appeared at all: the only witness from such a group, Barak Barfi of the generally left-of-center New America Foundation, made one appearance.

Since 2010, when the GOP retook the House, the Foreign Affairs Committee has been led by hard-liners. Florida’s über-aggressive Ileana Ros-Lehtinen was replaced last year by California’s Ed Royce, who is only slightly less extreme. In 2013, Royce’s committee unanimously approved legislation that the FDD helped write — and that AIPAC has backed — which would tighten the screws on Iran, giving “the ayatollah a choice between the collapse of his economy or compromise on his nuclear weapons program and giving up that program,” in Royce’s words. The bill came to a full House vote at the end of July, just days before Rouhani’s inauguration.

However, the determination by the White House to take advantage of Rouhani’s overtures brought about a remarkable change in Congress, stiffening the spines of liberal Democrats. Led by Jim McDermott, John Conyers, Keith Ellison and Jim McGovern, they issued a letter on July 31 urging delay, saying it would be “counterproductive and irresponsible to vote on this measure before Iran’s new president is inaugurated.” More than a quarter of the House — including more than 100 who would later vote for the bill Royce co-sponsored — signed another letter encouraging the Obama administration to explore diplomacy with Iran. “It would be a mistake,” warned the signatories, “not to test whether Dr. Rouhani’s election represents a real opportunity for progress toward a verifiable, enforceable agreement on Iran’s nuclear program.” Even so, Royce’s bill would pass by a lopsided 400-20 vote.

Nonetheless, by late November of last year, several rounds of intense negotiations yielded the Joint Plan of Action, which saw the United States and its partners — the four other permanent UN Security Council members plus Germany, called the P5+1 — modestly ease sanctions in exchange for Iran’s reducing its stockpiles of nuclear material and ceasing enrichment to higher levels. A key component of the interim deal was that the P5+1 would refrain from imposing new sanctions. The two sides agreed to aim at striking a final accord within six months — and both would need to keep their respective hardliners in check.

The Obama administration would soon face another hurdle with hawks on the Hill. In December, Senators Robert Menendez, Mark Kirk and Charles Schumer introduced their counterpart to the Royce bill. AIPAC threw its weight behind it, helping to amass fifty-nine co-sponsors. Though vaguely worded, the Senate bill sought to destroy negotiations by imposing deeper sanctions, insisting that any final deal must “dismantle Iran’s illicit nuclear infrastructure, including enrichment and reprocessing capabilities and facilities.”

The sanctions hardliners heaped praise on the measure. In a video posted online, AIPAC’s policy director, Brad Gordon, claimed the effort would “dramatically enhance our chance” of striking a deal with Iran and urged AIPAC’s members to write their senators to garner co-sponsors. But the bill raised alarms among advocates of diplomacy — including, most importantly, the White House. “This bill is in direct contradiction to the administration’s work to peacefully resolve the international community’s concerns with Iran’s nuclear program,” said National Security Council spokeswoman Bernadette Meehan. Obama issued a veto threat.

If there were any doubts about the danger that such a bill posed to an agreement, Iranian Foreign Minister Mohammad Javad Zarif laid them to rest when he was asked by Time what would happen if Washington enacted new sanctions — even those with a delayed trigger — during the talks. “The entire deal is dead,” Zarif responded.

With the wind in its sails, the White House got help from dovish groups like the Friends Committee on National Legislation and J Street, which rallied their supporters against the Menendez-Kirk-Schumer bill. Gould, the FCNL lobbyist, says members of Congress received more than 10,000 calls about it — “overwhelmingly pro-diplomacy,” she adds. Crucially, Senate majority leader Harry Reid refused to bring the bill to the floor for a vote.

Spooked, some of the sixteen Democratic co-sponsors begged off their initial support. As Senator Jeff Merkley put it in a letter to constituents, “At this time I do not support additional sanctions legislation because I share the views of many foreign policy experts that it could undermine the ongoing negotiations.” By February, AIPAC had also backed off, deploying influential constituents and donors to urge against a Senate vote, according to the Daily Beast. Even Menendez suggested that the time was not right to vote on his own bill. “While AIPAC is very powerful, it doesn’t win all the time,” says a former Obama administration official. “And because it’s misstepping so frequently, it’s losing quietly.”

With Democratic leaders averse to passing further legislation now, sanctions hawks are left with an increasingly partisan congressional base. But they remain undeterred. Should the P5+1 strike a deal with Iran, the hardliners are preparing new tactics to limit sanctions relief. Key to this strategy is shifting the goal posts. Right now, the White House is demanding that Iran reduce its uranium enrichment and allow inspectors greater access. The hawks would like to shift conventional wisdom — if not explicit policy — toward the goal of regime change. As Dubowitz put it bluntly at the FDD’s conference this spring, “The strategic problem here is the nature of the regime.”

At the end of June, Foreign Policy acquired an FDD document that it characterized as a “playbook for opposing an Obama nuclear deal with Iran.” The paper, written by Dubowitz and Senator Kirk’s recently departed deputy chief of staff, Richard Goldberg, calls on Congress to oppose the rollback of sanctions until Iran proves that its entire financial sector has no involvement in terrorism, money laundering or proliferation. By broadening the scope of requirements for sanctions relief, these demands would quickly render an otherwise historic deal into no deal at all. In July, Congress dutifully obliged: Royce, along with Democrat Eliot Engel, spearheaded a letter — signed by 342 of his colleagues — demanding a larger congressional role in talks and raising requirements on lifting sanctions to the FDD’s expansive standards. “Iran’s permanent and verifiable termination of all of these activities — not just some — is a prerequisite for permanently lifting most congressionally-mandated sanctions,” the letter stated.

The stakes, of course, are high. An agreement could open the door to a new era of improved relations with Iran and lead to desperately needed cooperation in solving what has become a regional sectarian war. The alternative is deepening confrontation — fueled by the same impulse that gave rise to the disastrous war in Iraq.

This story was reported in partnership with The Investigative Fund at The Nation Institute, now known as Type Investigations.