Inside a top-security building at a classified U.S. site, government experts intensely monitor rows of tall, cylindrical machines that may offer the Obama administration its best hope for persuading the public to back a nuclear deal with Iran.
Using centrifuges acquired when Libya abandoned its nuclear program in 2003, as well as American-built equipment, the government has spent millions of dollars over more than a decade to build replicas of the enrichment facilities that are the pride of Iran's nuclear program.
Since negotiations with Iran began in earnest, U.S. nuclear technicians have spent long hours tinkering with the machines to test different restrictions and see how much they would limit Iran's ability to convert uranium into bomb fuel.
Soon, the administration may be using the results of that secret research to try to convince the public that negotiations produced a good deal.
Diplomats from the United States, five other world powers and Iran appear to have made significant progress in their effort to reach the outline of a nuclear deal by the end of this month.
But the administration still faces a daunting task not just to complete the deal, but also to then sell it to the public and Congress. The facts accumulated over more than a dozen years by a small army of experts from the Energy Department and other agencies will play a key role in that effort, officials hope.
So far, "the opposition is winning the battle for public opinion," said Joseph Cirincione, president of the Ploughshares Fund, an arms control advocacy foundation and an administration ally. "They say we're working with the evil Iranians. They say Obama is vainglorious and weak."
But once the deal is complete and the administration lays out a detailed factual case, he predicted, "all that will shift dramatically."
U.S. officials won't comment on the classified research, which is being conducted at an undisclosed location in the United States. But former officials and private analysts say American agencies have constructed models of the Iranian facilities, relying on informants in Iran, information from foreign governments and voluminous data about Iran's program collected by the International Atomic Energy Agency, or IAEA, the United Nations' nuclear watchdog.
The technical data from that modeling effort provide insights on whether any deal will accomplish the main Western goal: to prevent the Iranians from being able to accumulate the amount of fissile material that would allow them to build a single bomb in a year or less.
The United States, France, Britain, Germany, Russia and China are trying to work out such a deal. They would ease tough economic sanctions on Iran in return for Tehran's agreement to restrict its nuclear activities for 10 to 15 years. If the negotiators can reach the outline of an agreement this month, they will try to complete a comprehensive agreement before the end of June.
The six world powers are trying to hold down Iran's potential "breakout" period — the amount of time that would be needed to produce enough fissile material for a single warhead — by limiting the number of centrifuges Iran can have, capping its stockpile of enriched uranium and restricting the way it connects hundreds of centrifuges into groups called cascades.
Under an interim agreement that the six world powers and Iran reached in November 2013, Iran agreed to eliminate its stockpile of uranium that had been enriched to contain 20% fissionable material — a level that is short of weapons-grade but that can fairly quickly be enriched further. The material was diluted or converted to a form that can't readily be enriched to a higher level.
The agreement set a cap on the amount of uranium enriched to 3.5% that the Iranians can keep. The country's nuclear sites are monitored on a daily basis by the IAEA to ensure compliance. As a result, although Iran continues to spin its centrifuges, it is further away from having a weapon than before the interim agreement was reached, according to administration officials. Israeli officials, who oppose the longer-term deal taking shape, have generally agreed that the interim deal has worked.
In its quest for public support of the overall deal, the White House has some catching up to do. Officials had hoped to be able to delay debate on a deal until an agreement was complete.
But as talks bogged down last year, with two deadlines missed, critics of the deal mobilized and began attacking details of the negotiations that leaked into public view, in part from Israeli officials who oppose the potential pact. Critics include skeptical lawmakers, foreign governments, particularly Israel, and a panoply of well-funded lobbying and research groups in Washington.
Israeli Prime Minister Benjamin Netanyahu, in his March 3 address to Congress, roundly criticized the proposed deal because it reportedly might last only 10 years and would allow Iran to retain 6,000 or more centrifuges, compared with the few hundred that the administration originally demanded.
Administration officials say that in return for agreeing to a larger number of centrifuges, they have insisted that Iran accept other restrictions. Those would cap the stockpile of enriched uranium in Iran and restrict the configuration of cascades to reduce their output.
Officials say they are confident the public would prefer a deal to the uncertain alternatives of war or more economic sanctions. But the White House has shown it takes the criticism seriously by organizing a campaign-style communications effort that tries to respond quickly to attacks.
President Obama's inner circle meets several times a week to organize the effort to win over public opinion for what is one of the top goals of his final term. The aides plan to bolster their efforts by lining up endorsements from respected public figures, nuclear specialists and allied governments.
The public appears ambivalent. Several polls have indicated that Americans would prefer a negotiated solution, but a Wall Street Journal-NBC News poll released March 9 suggested that 71% doubted a deal would keep Iran from eventually getting the bomb.
The administration gained ground last week from a Republican blunder when 47 GOP senators signed a letter to Iran warning that Congress could torpedo any deal and that a future president could abandon it. The letter set off an angry Democratic backlash and, according to lawmakers, lowered the chances that Democratic senators would provide enough votes to allow victories for Republican bills that the White House fears would undermine the talks.
One of the proposed bills would add new sanctions if there's no deal. Administration officials say passage of more sanctions now, even ones contingent on future events, would torpedo the talks. The other bill would subject any Iran deal to an up-or-down vote in Congress. Unlike a treaty, the Iran deal would be an executive agreement, which does not require Senate ratification. Congress would still have to vote on measures to lift some existing sanctions although others could be modified by the president.
"Selling the deal in Congress is going to be a major challenge," said Rep. David E. Price (D-N.C.), a supporter of the diplomacy. "No question about it."
The administration's arguments for the deal may also be challenged by outside experts. Israel also might weigh in; analysts say the country has built its own models of the Iranian hardware.
Gary Samore, Obama's top arms control advisor in the first term, said administration officials are "in a very strong position to make technical arguments to defend their case that they've got a one-year breakout period."
But "all these calculations are based on uncertainties and assumptions, so there is room for other well-informed people to challenge them," said Samore, now executive director for research at the Belfer Center for Science and International Affairs at Harvard's Kennedy School of Government.
It's still unclear whether the anticipated framework, if reached this month, would be a detailed document or a vague one that is harder to defend.
Iran's supreme leader, Ayatollah Ali Khamenei, has said he wants no written agreement until all details are settled. And the two sides remain divided on crucial issues, including the pace of sanctions relief, limits on Iranian nuclear research and Iran's enrichment capacity and monitoring of Iran's program, diplomats say.