Tehran’s demand that the United States lift its designation of the Iranian Revolutionary Guard Corps as a foreign terrorist organization, and U.S. refusal so far to do that, have brought the year-long negotiations over reviving the Iran nuclear deal to a halt, with no new meetings scheduled and little obvious room for compromise.

Since talks being held in Vienna adjourned last month, European participants have shuttled between Washington and Tehran in a vain search for accommodation from both sides. “At this point, nothing mutually acceptable” has been proposed, according to a U.S. official knowledgeable about the issue who, like others, spoke on the condition of anonymity to discuss the sensitive diplomatic and political matter.

Whether the United States will yield in any way is up to President Biden, and “the president hasn’t made a decision,” the official said. “Politically, we know that it’s an extremely difficult step to take.” For the moment, head U.S. negotiator Robert Malley said at a foreign policy forum last weekend, success “is not just around the corner, and not inevitable.”

Those beyond the inner circles in the two capitals are growing increasingly worried. “We must conclude this negotiation. Much is at stake,” Enrique Mora, the European Union deputy foreign policy chief wrote on Twitter last week before visits to both Washington and Tehran early this week. The E.U. is coordinating the talks between Iran and other signers of the original 2015 deal, which are Britain, France, Germany, Russia and China.

Although the talks essentially involve Iran and the United States, which withdrew from the agreement in 2018, Tehran has refused to speak directly to the Biden administration, which is technically only an indirect participant, speaking to Iran through the Europeans.

While there is a handful of other, less contentious issues still to be settled, people familiar with the matter indicated that disagreement now revolves almost entirely around the Revolutionary Guard Corps issue. Earlier U.S. consideration of a public Iranian statement disavowing regional aggression, in exchange for the terror delisting, is no longer on the table, the people said.

For the administration, the biggest impediment to lifting the designation is the likely reaction in Congress, where the delisting issue has only increased considerable bipartisan opposition to any revived agreement with Iran. In briefings and meetings with lawmakers in recent weeks, Malley and Brett McGurk, the Middle East coordinator at the National Security Council, have outlined what they believe would be the negligible effect of the delisting, as well as the peril of not concluding a deal.

The Revolutionary Guard Corps and its leaders are under a wealth of other State and Treasury Department terrorism sanctions that would remain in place, including Iran as one of four countries on the official U.S. list of state sponsors of terrorism. Regardless of what happens in the negotiations, Malley said at the weekend conference in Qatar, the Revolutionary Guard Corps “will remain sanctioned under U.S. law and our policies, and our perceptions” of it “will not change.”

Speaking at the same forum, Kamal Kharazi, former Iranian foreign minister and current adviser to Supreme Leader Ali Khamenei, said the Revolutionary Guard Corps “certainly must be removed” from the foreign terrorist organization list. When Foreign Minister Hossein Amir Abdollahian indicated some flexibility in a recent television interview, he quickly backtracked, indicating in an Instagram post that the issue was a “red line” for Iran.