Israel's Prime Minister Benjamin Netanyahu speaks in Jerusalem on Feb. 16. (Ronen Zvulun/Reuters)

On Tuesday, Israeli Prime Minister Benjamin Netanyahu will be in Washington, trying to spur Congress to cut short what he considers a feeble and dangerous deal in the works over Iran’s nuclear program.

Simultaneously, as if on a split screen, Secretary of State John F. Kerry will be in Montreaux, Switzerland, trying to nail down a historic accord that could give the world a year to react if Iran were to stockpile nuclear materials for a bomb and that could wean Iran away from international pariah status.

The tension between those two competing worldviews on Iran — one judging the risks too great to take and the other finding a greater risk in walking away from a deal — has persisted for years. But it has reached an apex for a simple reason: Iran and the United States, plus its five negotiating partners, appear closer to a deal than at any time in more than a decade of talks.

“The achievements to date are so huge, unprecedented really, that it is very, very, very difficult for both parties to leave the negotiating table and go backward,” said Hossein Mousavian, an Iranian diplomat who once was spokesman for his country’s nuclear negotiating team and is a visiting scholar at Princeton University.

In his speech to a joint session of Congress, Netanyahu will elaborate on the alarm he has raised frequently — Iran cannot be trusted to keep its word and the still-incomplete deal poses an existential threat to Israel and the world at large.

Graphic: Iran's potential nuclear capability

The ramifications of no deal, however, are also perilous. Congress would all but certainly impose more sanctions on an Iranian economy that has already been buffeted by harsh financial measures. Other countries could ignore the call to continue isolating Iran economically, unraveling a united diplomatic front that the Obama administration has worked strenuously to maintain.

Iran would likely ramp up uranium enrichment, bringing it closer to the bombmaking capability that world powers have worked for so long to deny it. The United States would consider launching a military strike to prevent Iran from obtaining a nuclear weapon.

“One question that I hope both Republicans and Democrats ask Netanyahu while he’s here is to be clear about what alternative is available to us,” said Gary Sick, who served on the National Security Council under both Republican and Democratic administrations and is a scholar at Columbia University's Middle East Institute. “It’s one thing to say we’d like to have an agreement in which Iran has no capability of building a nuclear weapon, under any circumstances, at any time in the future. But that is by definition impossible.”

The talks with Iran have been grinding on since President George W. Bush’s first term. The West wants to monitor and curtail Iran’s nuclear program, which began under the shah with help from the United States under an Eisenhower-era program called Atoms for Peace.

Iran insists that it uses its nuclear technology only for peaceful purposes, such as medical testing and energy, but many in the West fear that the authoritarian government aims to stockpile high-level nuclear material and eventually produce nuclear weapons.

Although Iran kept building up its nuclear capacity for much of the past decade, talks over its program gained new momentum after the 2013 election of President Hassan Rouhani, who campaigned on a promise to get Iran out from under the yoke of sanctions. Since then, a temporary agreement has frozen and contracted Iran’s nuclear output and development while Tehran negotiates with the United States and five world powers — Britain, France, China, Russia and Germany.

The provisional pact, extended twice, remains in effect through June 30, the hard deadline. But the parties have given themselves until the end of March to agree on a framework, leaving the remaining time to work out complex technical details.

Jason Rezaian’s journey has taken him from a childhood in San Francisco to his father’s native Iran. At 37, he became the Washington Post correspondent in Tehran. In July 2014, he was thrown into Iran’s Evin Prison, where he remains, without access to a lawyer. This is his story. (Jorge Ribas/The Washington Post)

A senior State Department official, speaking Friday to reporters on the condition of anonymity under rules of protocol, assessed the odds of cinching a deal at 50-50.

“Iran has to make a significant political decision to allow the flexibility to close this deal,” the official said. “That's the unknowable variable.”

Mousavian, who maintains contact with negotiators, said the parties have settled three main sticking points. Iran will not enrich uranium beyond 5 percent, much lower than higher-grade uranium that can be purified quickly into weapons grade. It also has agreed to reduce levels of plutonium — which could provide another pathway to a bomb — at a heavy-water facility near Arak. And it will continue sending spent nuclear fuel to Russia for reprocessing rather than doing the work in Iran.

In addition, Mousavian said, Iran is willing to convert the Fordow uranium-enrichment plant near Qom to a research and development center, but not close it down completely.

Taken together, he said, the changes would significantly increase the so-called breakout time needed to amass enough material for a bomb. Now, official intelligence estimates are that Iran could do it within two or three months. The talks aim to expand it to a year or longer.

The remaining gaps concern confidence-building measures that would dictate the pace at which sanctions and restrictions are eased in the latter years of an agreement, Mousavian said.

“Some of the key elements appear to be falling into place,” said Robert Einhorn, a senior fellow with the Arms Control and Non-Proliferation Initiative at the Brookings Institution. “But many of the showstopping details have yet to be worked out, including the question of monitoring arrangements, which have to be very rigorous.”

If the administration cannot demonstrate meaningful progress, Einhorn said, many in Congress will conclude that Iran has no intention of making concessions, and demand an end to talks.

“On the other hand, there will be strong pressure to continue if there’s been fundamental progress on some central issues like enrichment capacity, duration and the phasing of sanctions relief,” Einhorn said. “If those issues are resolved, there will be strong pressure to continue working at the details till they get it right.”

Gary Samore, a former State Department official who is head of research at the Belfer Center for Science and International Affairs at Harvard’s Kennedy School, said tolerance for a third extension is thin.

“Obama may be able to buy some time by asking for negotiators to have a couple more weeks to finish the political framework,” said Samore, who is president of a group called United Against Nuclear Iran. “But they’ve pretty much run out of time.”

That would not necessarily signify an abrupt end to negotiations. Obama has the authority to waive more sanctions that many in Congress want to impose; Rouhani in turn could waive a resumption in nuclear activity, although only with the backing of Iran’s supreme leader, the ultimate arbiter of the country’s nuclear decisions.

Still, if political pressure in both capitals causes the interim agreement to collapse, the consequences could rapidly worsen.

“We’d emphasize the resumption of sanctions and economic pressure,” said Samore, predicting the United States would pressure Iran’s main oil customers — Japan, Korea, India and China — to reduce their purchases. “The Iranians would resume nuclear activities. But they’d be very careful about doing anything that would trigger an American or an Israeli attack. Neither wants to get into a war right now.”

Diplomatic pressure would likely intensify. If the United States is perceived as rejecting a deal acceptable to the other five nations that are its negotiating partners, Iran will argue it should not be blamed.

Sick, who served on the National Security Council, outlined a scenario in which the United States is portrayed as the culprit for a broken deal and ends up alone in sanctioning Iran as other countries return to doing business with Tehran. If Iran were to start making enriched uranium at weapons-grade levels, the possibility of a Israeli military strike would rise. And if the United States wanted to negotiate with Iran under those circumstances, it would be in a weaker position to extract Iranian concessions.

“When you look at the choices, not fantasy alternatives but real alternatives, it’s not a very attractive picture,” Sick said. “It’s a tremendously dangerous moment.”

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