By Laurence Norman

Foreign-policy chief Josep Borrell sees no good alternative to reviving the nuclear deal even as the Biden administration shifts focus

KYIV, Ukraine—The European Union’s top diplomat, Josep Borrell, is refusing to give up on efforts to rescue the 2015 Iranian nuclear deal, even as Tehran cracks down on protesters at home and helps Russia in its war against Ukraine.

On a secure train returning from an EU leaders trip to Kyiv, Mr. Borrell told The Wall Street Journal that critics of his efforts to revive the pact perhaps “don’t value enough” the dangers of a nuclear Iran.

“As far as I know, there is not an alternative to this deal to try to avoid Iran becoming nuclear,” he said.With the Biden administration appearing to be increasingly indifferent to the accord’s fate, Mr. Borrell—who is the coordinator of the nuclear negotiations—has found himself under attack from opponents of the pact. They see him as personifying Europe’s attachment to the deal, which lifted most international sanctions on Iran in exchange for strict but temporary restrictions on Tehran’s nuclear program.

France, Britain and Germany—who negotiated the 2015 deal alongside the U.S. with Iran—remain supportive of reviving the deal. However most European diplomats are gloomy about the agreement’s survival.

The 75-year-old veteran Spanish politician is also under pressure from Tehran. Iran has warned that EU sanctions on Iran and a German proposal to list Iran’s Islamic Revolutionary Guard Corps as a terrorist organization could lead to a collapse in ties and risks to European security.

Mr. Borrell, who has sometimes landed himself in trouble for his plain speaking, insists his role as coordinator of the nuclear diplomacy doesn’t stop him pressing Tehran over its actions at home or abroad. He speaks regularly to Iranian Foreign Minister Hossein Amir-Abdollahian.

“Look, I can tell you my last conversation: Stop capital punishment. Stop repression…So if you continue on this way, you will make (it) impossible to have any kind of political arrangement.”

Mr. Borrell’s claim that the 2015 agreement is the only way to stop Iran going nuclear is one that opponents of reviving the dispute over the nuclear deal. In the U.S., the ranks of skeptics about the diplomacy have swelled recently to include some people who strongly supported the deal in 2015.

Some sanctions on Iran are already starting to expire and from 2025 to 2030, key restrictions will lift on Iran’s production and stockpiling of enriched uranium. Opponents of the deal argue that it would clear the way for Tehran to eventually obtain nuclear weapons. Iran insists its nuclear work is entirely peaceful.

The odds of reviving the deal currently appear to be minimal. Former President Donald Trump in 2018 withdrew the U.S. from the deal, saying it was “a horrible, one-sided deal” that wouldn’t block Tehran from developing nuclear weapons.

Iran later started to breach almost every limit on its activities under the accord. It is currently producing high-grade 60% uranium—the only nonnuclear weapon state to do so. U.N. atomic agency chief Rafael Grossi said last month that Iran has enough enriched uranium for “several nuclear weapons.”

The Biden administration has set reviving the 2015 deal as a key foreign-policy goal. A deal was close last spring, but it stalled. New talks in August in Vienna produced what Mr. Borrell’s team called a final draft text for Tehran and Washington to accept. Iran at first seemed close to agreeing but then set new demands on the deal, and requested a condition that a U.N. atomic-agency probe into its nuclear activities be stopped.

Days later, mass protests started in Iran over the killing of a young woman, Mahsa Amini, in police custody and Iranian authorities responded with violence. Meanwhile, Russia had started deploying Iranian drones against Ukraine, triggering new Western sanctions against Tehran.

U.S. officials warned last fall that Iran intends to deliver ballistic weapons to Russia. In mid-January, the American ambassador to the U.N., Linda Thomas-Greenfield, said that Iran “is now considering” the sale of hundreds of missiles to Russia.

Doing so, Mr. Borrell warned, “would be a step too far” but he thinks Tehran now understands that. Some Western diplomats say there appears to be a debate within Iran about what to do.

“I have to say that every time I talk with the Iranians, they insist that they will not do it,” Mr. Borrell said of Iran selling missiles to Russia. “But then I have to advise them, if you do that, everything will be much more difficult, including the JCPOA,” he said, using the acronym for the nuclear deal, the Joint Comprehensive Plan of Action.

Mr. Borrell, a senior Socialist party politician in Spain, is frank that given Iran’s domestic crackdown and Tehran’s support for Russia, he doesn’t know if Washington would agree to revive the nuclear deal now if Iran scaled back its deal-related demands.

U.S. officials have been saying since the protests erupted in September that Washington’s focus is on the domestic situation in Iran and not the talks. But they haven’t ruled out a diplomatic revival.

Mr. Borrell said that if Iran took a further step in its military assistance to Russia, the incentive for Europe and the U.S. to restore the deal “would be much less.”

So far, Tehran has steered clear of producing 90% weapons-grade material, a step Israel and some Western officials believe should be the threshold for a military response. Last week, the U.N. atomic agency reported that Iran had made changes to the set up of its centrifuges that produce 60% enriched uranium without reporting it to them.

Two Western diplomats said that Iran may have been experimenting with potential ways to produce weapons-grade material. On Friday, the U.S. and three European allies condemned Iran’s move, dismissing Tehran’s explanation that it didn’t intend to conduct the work.

In ending the deal in 2018, Mr. Trump said Iran had lied about the peaceful nature of its nuclear work and offered large economic benefits to a state-sponsor of terrorism. He said he would negotiate a broader, tougher deal.

Mr. Borrell refuses to pin the blame for the nuclear deal’s woes on Tehran or the Biden administration. But he offered a terse response when asked who was responsible for the agreement’s near-collapse.

“Trump,” he said.