U.S. and Iran Want to Restore the Nuclear Deal. They Disagree Deeply on What That Means.

President Biden and Iran’s leaders say they share a common goal: They both want to re-enter the nuclear deal that President Donald J. Trump scrapped three years ago, restoring the bargain that Iran would keep sharp limits on its production of nuclear fuel in return for a lifting of sanctions that have choked its economy.

But after five weeks of shadow boxing in Vienna hotel rooms — where the two sides pass notes through European intermediaries — it has become clear that the old deal, strictly defined, does not work for either of them anymore, at least in the long run.

The Iranians are demanding that they be allowed to keep the advanced nuclear-fuel production equipment they installed after Mr. Trump abandoned the pact, and integration with the world financial system beyond what they achieved under the 2015 agreement.

The Biden administration, for its part, says that restoring the old deal is just a steppingstone. It must be followed immediately by an agreement on limiting missiles and support of terrorism — and making it impossible for Iran to produce enough fuel for a bomb for decades. The Iranians say no way.

Now, as negotiators engage again in Vienna, where a new round of talks began on Friday, the Biden administration finds itself at a crucial decision point. Restoring the 2015 accord, with all its flaws, seems doable, interviews with European, Iranian and American officials suggest. But getting what Secretary of State Antony J. Blinken has called a “longer and stronger” accord — one that stops Iran from amassing nuclear material for generations, halts its missile tests and ends support of terrorist groups — looks as far away as ever.

That is potentially a major political vulnerability for Mr. Biden, who knows he cannot simply replicate what the Obama administration negotiated six years ago, after marathon sessions in Vienna and elsewhere, while offering vague promises that something far bigger and better might follow.

Iran and the United States “are really negotiating different deals,” said Vali R. Nasr, a former American official who is now at Johns Hopkins University School of Advanced International Studies. “It’s why the talks are so slow.”

The Americans see the restoration of the old deal as a first step to something far bigger. And they are encouraged by Iran’s desire to relax of a series of financial restrictions that go beyond that deal — mostly involving conducting transactions with Western banks — because it would create what one senior administration official called a “ripe circumstance for a negotiation on a follow-on agreement.”

The Iranians refuse to even discuss a larger agreement. And American officials say it is not yet clear that Iran really wants to restore the old deal, which is derided by powerful hard-liners at home.

With Iran’s presidential elections six weeks away, the relatively moderate, lame-duck team of President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif are spinning that an agreement is just around the corner. “Almost all the main sanctions have been removed,” Mr. Rouhani told Iranians on Saturday, apparently referring to the American outline of what is possible if Tehran restores the sharp limits on nuclear production. “Negotiations are underway for some details.”

Not so fast, Mr. Blinken has responded. He and European diplomats underscore that Iran has yet to make an equally detailed description of what nuclear limits would be restored.

But even if it does, how Mr. Biden persuades what will almost surely be a new hard-line Iranian government to commit to further talks to lengthen and strengthen the deal is a question American officials have a hard time answering. But Mr. Biden’s aides say their strategy is premised on the thought that restoring the old deal with create greater international unity, especially with Europeans who objected strenuously to Mr. Trump’s decision to exit a deal that was working. And even the old deal, one senior official said, “put a serious lid on Iran’s nuclear program.”

Hovering outside the talks are the Israelis, who continue a campaign of sabotage and assassination to cripple the Iranian program — and perhaps the negotiations themselves. So it was notable that the director of the Mossad, who has led those operations, was recently ushered into the White House for a meeting with the president. After an explosion at the Natanz nuclear plant last month, Mr. Biden told aides that the timing — just as the United States was beginning to make progress on restoring the accord — was suspicious.

The split with Israel remains. In the meetings in Washington last week — which included Mr. Blinken; the C.I.A. director, William J. Burns; and the national security adviser, Jake Sullivan — Israeli officials argued that the United States was naïve to return to the old accord, which they think preserved a nascent nuclear breakout capability.

Mr. Biden’s top aides argued that three years of “maximum pressure” on Iran engineered by Mr. Trump and his secretary of state, Mike Pompeo, had failed to break its government or limit its support of terrorism. In fact, it had prompted nuclear breakout.

In Vienna, by all accounts, the lead negotiator, Robert Malley — whose relationship with Mr. Blinken goes back to the high school they attended together in Paris — has made a significant offer on lifting sanctions “inconsistent” with the original deal.

On Wednesday, Mr. Blinken said that the United States had “demonstrated our very seriousness of purpose” in returning to the deal.

“What we don’t yet know is whether Iran is prepared to make the same decision and to move forward,” he told the BBC.

Iran wants more sanctions lifted than the United States judges consistent with the deal, while insisting on keeping more of its nuclear infrastructure — in particular advanced centrifuges — than that deal permits. Instead, Iran argues that the International Atomic Energy Agency should simply inspect the new centrifuges, a position that is unacceptable to Washington.

While the talks continue, Iran is keeping up the pressure by adding to its stockpile of highly enriched uranium and the equipment to make it, all in violation of the deal.

Both Iran and the United States are working under delicate political constraints. Even as Iran’s supreme leader, Ayatollah Ali Khamenei, has supported the Vienna talks, Mr. Rouhani and Mr. Zarif are mocked by powerful conservatives who do not trust Washington and who expect to capture the presidency.

For his part, Mr. Biden must contend with a Congress that is highly skeptical of a deal and largely sympathetic to the concerns of Israel.

But with the Iranian elections close, time is pressing, and the Biden administration lost significant chunks of it as its negotiating position has evolved, officials say. The Americans initially demanded that Iran return to compliance, and then chose to keep some of the Trump administration’s sanctions in place as leverage to try to force a broader negotiation.

In two discussions in February, the Europeans urged American officials to start negotiating in earnest and lift some sanctions as a gesture of good faith toward Iran. Those suggestions were ignored. But when Ayatollah Khamenei said that the country could proceed to enrich uranium up to 60 percent purity — as opposed to the 3.67 percent limit in the nuclear deal — Washington took matters more seriously, officials said, fearing that it would further diminish the so-called breakout time for Iran to get enough material for a bomb.

It was only at the end of March that the two sides agreed to negotiate the whole deal at once, and the Vienna talks began in early April. Then it took more time for the Americans to concede that returning to the 2015 deal as it was written was the best and perhaps only way to build enough trust with Iran that its leaders might even consider broader, follow-on talks.

Three working groups have been established: one to discuss which sanctions Washington must lift, one to discuss how Iran returns to the enrichment limits and one to discuss how to sequence the mutual return. Iran has not yet engaged seriously on its plans, still insisting that Washington move first, but another sticking point remains: which sanctions will be lifted.

Mr. Trump restored or imposed more than 1,500 sanctions in an effort to prevent a renewal of the pact. The sanctions have been put into three baskets — green, yellow and red, depending on how clearly they are inconsistent with the deal. Green will be lifted; yellow must be negotiated; and red will stay, including, for example, sanctions on individuals for human-rights violations.

Deciding which sanctions to lift is politically delicate for both countries. For example, in the yellow category, Iran insists that a Trump-era sanction of its central bank under a terrorism designation must be lifted because it damages trade. But it would be even more complicated for Washington to lift the terrorism designation on the powerful Islamic Revolutionary Guards Corps, the officials said.

For the Iranians to agree to a deal that does not resolve the designation of the Guards would be a hard sell, even for the supreme leader.

“For Biden, it’s hard to justify lifting sanctions against institutions still threatening U.S. interests in the region, and it’s hard for Rouhani to go home boasting about lifting all sanctions except those on his rivals,” said Ali Vaez, the Iran project director at the International Crisis Group.

“It’s a fragile process,” Mr. Vaez said, noting Iran’s rocket attacks in Iraq. “If a single American is killed, the whole process is derailed.”

But how Mr. Biden gets Iran to move to negotiate a better or new accord is the question.

American officials have no real answer to this dilemma as they try to resurrect the old deal, but they assert that Iran, too, wants more benefits than the old deal provided, so it should be willing to talk further. The Americans say they are ready to discuss how to strengthen the deal to mutual benefit, but they say that would be a decision for Iran to make.

Despite Iran’s pressure tactics — increasing enrichment to just short of bomb grade in small quantities and barring international inspectors from key sites in late February — Mr. Zarif insists that these moves are easily reversible.

American intelligence officials say that while Iran has bolstered its production of nuclear material — and is probably only months from being able to produce enough highly enriched uranium for one or two bombs — even now, there is no evidence Iran is advancing on its work to fashion a warhead. “We continue to assess that Iran is not currently undertaking the key nuclear weapons-development activities that we judge would be necessary to produce a nuclear device,” Avril D. Haines, the director of national intelligence, said in a report last month.

The Israelis are more skeptical, arguing that evidence they stole from a warehouse “archive” of Iran’s nuclear program three years ago shows that Iranian scientists had already done extensive work on warhead design.

Mr. Blinken says that the Vienna talks are intended to return to the stability and oversight of Iran’s nuclear program that the 2015 deal provided until it was abandoned by Mr. Trump.

“So there’s nothing naïve about this. On the contrary, it’s a very cleareyed way of dealing with a problem that was dealt with effectively by the J.C.P.O.A.,” Mr. Blinken said, referring to the 2015 deal. “We’ll have to see if we can do the same thing again.”

The atmosphere in Iran has been complicated by a recent scandal over Mr. Zarif, whose criticism of internal decision-making recently leaked, apparently in an effort to damage his reputation and any chance he had to run for the presidency.

Ayatollah Khamenei refuted the criticism without naming Mr. Zarif, but he said the comments were “a big mistake that must not be made by an official of the Islamic Republic” and “a repetition of what Iran’s enemies say.”

At the same time, by downplaying Mr. Zarif’s role, the supreme leader reaffirmed his support for the talks while also sheltering them from criticism by hard-liners, said Ellie Geranmayeh of the European Council on Foreign Relations.

Steven Erlanger reported from Brussels, and David E. Sanger from Washington. Farnaz Fassihi contributed reporting from New York.