LAUSANNE, Switzerland — At the Massachusetts Institute of Technology in the mid-1970s, Ernest J. Moniz was an up-and-coming nuclear scientist in search of tenure, and Ali Akbar Salehi, a brilliant Iranian graduate student, was finishing a dissertation on fast-neutron reactors.


The two did not know each other, but they followed similar paths once they left the campus: Mr. Moniz went on to become one of the nation’s most respected nuclear physicists and is now President Obama’s energy secretary. Mr. Salehi, who was part of the last wave of Iranians to conduct nuclear studies at America’s elite universities, returned to an Iran in revolution and rose to oversee the country’s nuclear program.


Forty years later, they are facing off in intense one-on-one talks as the deadline approaches for a nuclear deal that could be one of the most important, and disputed, international accords in decades.


Mr. Moniz and Mr. Salehi have emerged as their countries’ No. 2 negotiators, a pair of atomic diplomats taking on the vast technical issues that lie beneath the political disagreements. Their roles as deputies to the chief negotiators — Secretary of State John Kerry and Mohammad Javad Zarif, the Iranian foreign minister — signal that the two sides are down to the hardest issues about the kind of nuclear infrastructure Iran will be permitted to retain, and in recent days, those discussions have hit major road blocks.


The two had met only once before, in Vienna, more than a decade ago. But in the last five weeks, they have negotiated alone for more than 20 hours.


“We have a good rapport,” Mr. Moniz said as he poured himself a glass of well-aged Scotch and settled into the living room of his Lausanne suite, overlooking Lake Geneva. Over time, he said, Mr. Salehi has dropped his formality (Mr. Moniz calls him Ali), and the two now disappear for hours at a time into the conference rooms at the Beau-Rivage Palace Hotel, where the dismantlement of the Ottoman Empire was negotiated nearly a century ago.


The question is whether it is possible to dismantle enough of Iran’s nuclear infrastructure to assure the United States and its allies that they would have enough warning to stop Iran if it tried to build a nuclear bomb.


Mr. Moniz, 70, understands his role well: He is providing not only technical expertise but also political cover for Mr. Kerry. If a so-called framework agreement is reached in the next few days, it will be Mr. Moniz who will have to vouch to a suspicious Congress, to Israel and to Arab allies that Iran would be incapable of assembling the raw material for a single nuclear weapon in less than a year.


“It wouldn’t mean much coming from Kerry,” said a member of the administration deeply involved in the strategy who spoke on the condition of anonymity. “The theory is that Ernie’s judgment on that matter is unassailable.”


Mr. Salehi, 66, will have his own problems selling an agreement to the generals and clerics in Tehran, many of whom are suspicious of Iran’s Western-educated negotiators and will have to be convinced that Iran has not backed down in the face of American demands. In recent days, Mr. Salehi has taken a decidedly positive tone in public, suggesting that all technical disputes with the United States have been resolved — a move some American officials interpret as an effort to put the blame squarely on Washington if the talks fail.


“For both sides, there are big questions of optics and politics here,” Mr. Moniz said.


People who know both men say they have more in common than challenges at home.


They are “mirror images in different contexts, both very personable,” said Michael J. Driscoll, an emeritus professor of nuclear science and engineering at M.I.T. who advised Mr. Salehi on his dissertation. Over the years, Mr. Salehi fell out of contact with Mr. Driscoll and many of his M.I.T. colleagues, who were warned after the 1979 Iranian revolution, which swept out the Washington-supported shah and brought to power a virulently anti-American Islamic leadership, that corresponding with Mr. Salehi could place him in jeopardy.


Mr. Salehi and Mr. Moniz converged a little more than a month ago in the increasingly tense talks, brought together after the Iranians announced that Mr. Salehi, a former foreign minister who represented Iran at the International Atomic Energy Agency in Vienna, would have a seat at the negotiating table. It was a telling move: Mr. Salehi is considered close to Iran’s supreme leader, Ayatollah Ali Khamenei, and behind the scenes in Tehran, he had just killed an American proposal to reconfigure Iran’s centrifuges in a way that would have made them far less capable of producing enriched uranium.


Soon, Mr. Moniz received a call from the White House: He would become Mr. Kerry’s negotiating partner.


Mr. Moniz was well suited for the job: After becoming energy secretary nearly two years ago, he brought in scientists from the United States’ national laboratories to work out options to present to the Iranians, based in part on a secret replica of Iran’s facilities that the United States built when it was mapping out cyberattacks against them during the Bush administration.


“Ernie loves to sit with these technical experts and challenge their conclusions,” said Elizabeth Sherwood-Randall, the deputy energy secretary. “I often see him doing the math in his head.”


Mr. Moniz’s arrival has, by all accounts, changed the dynamic of the negotiating sessions. “Occasionally, a scientist drops into the government at just the right time, with just the right expertise,” said John M. Deutch, a former director of the Central Intelligence Agency who appointed Mr. Moniz years ago as the head of M.I.T.’s nuclear physics laboratory. “And that’s what’s happened here.”


The Moniz-Salehi talks are a way to separate the heated arguments over sanctions and national sovereignty from a far more technical discussion that only two nuclear scientists could love.


They have spent much of their time in recent days arguing about the type and power of the advanced centrifuges Iran says it wants to continue developing during the 10 or more years of an agreement — one of the last stumbling blocks in the talks.


“We spend a lot of time on SWU,” Mr. Moniz said, referring to separative work units — the acronym is pronounced “swoo” in nuclear-speak — which underlie all the calculations about how long it would take Iran to produce a single bomb’s worth of enriched uranium. It is not the favorite subject of many State Department diplomats.


Mr. Moniz has also reached out to his vast network of nuclear scientists in the United States, giving them classified briefings about the details of the talks. His hope is that they will provide technical validation to Congress and nervous allies that the plan negotiated with Tehran will give enough warning time to head off an Iranian race for a nuclear weapon with economic pressure or, if need be, a bombing run.


Mr. Moniz, who was born in 1944 in Fall River, Mass., got hooked on science as a high school student in the post-Sputnik era of the late 1950s and early 1960s. After attending Boston College, he earned a doctorate in theoretical physics from Stanford and then he joined the faculty at M.I.T., where he fell in with a group of physicists who were active in the Union of Concerned Scientists and similar groups.


He soon found himself immersed in questions about managing the “nuclear fuel cycle” technology that was giving emerging nations the capability to build power reactors — and nuclear weapons. The spread of that bomb-making technology was an unintended consequence of President Dwight D. Eisenhower’s “Atoms for Peace” program. Mr. Moniz has spent the rest of his career trying to undo the damage.


“What couldn’t be realized back then,” he said, “was how fast the technology would create ‘threshold states’ ” on the verge of nuclear weapons. Iran is the classic example.


Mr. Salehi, who was born in 1949 in Iraq, studied in Lebanon at the American University of Beirut and went on to become chancellor of Iran’s Sharif University of Technology. Britain and other European nations have put him on sanctions lists, and he has been dogged by suggestions that he knew of shipments of banned technologies to Iran. When M.I.T. invited him to speak on campus a number of years ago, the State Department blocked his appearance, saying he could not leave the neighborhood near the United Nations in New York.


This weekend, Mr. Salehi and Mr. Moniz are talking once again, mostly to resolve the dispute about whether research on new centrifuges will take Iran too close to bomb-making capability.


But they are clearly bound by an old-school tie: Mr. Salehi recently became a grandfather for the first time, and Mr. Moniz showed up with baby gifts, each embossed with the M.I.T. logo.


By The New York Times