But that outcome, which would likely terminate the agreement, is precisely what the Europeans say they are trying to avoid. There is nervousness that Prime Minister Boris Johnson of Britain, with his eye on a post-Brexit trade deal with Washington, might crack, but European officials consider that possibility unlikely.

Mr. Johnson and his government support the nuclear deal and collaborated on a statement with France and Germany on Tuesday that said “we are not joining a campaign to implement maximum pressure on Iran.” On Tuesday, Mr. Johnson urged new talks with Washington and Tehran to try to negotiate a “Trump deal” to supplant the current one.

Under the dispute mechanism, explained Ellie Geranmayeh, an Iran expert with the European Council on Foreign Relations, time limits on discussions can be extended by consent. Any party to the deal can go directly to the United Nations Security Council to request the re-imposition of United Nations sanctions, but no one is expected to do that, unless Mr. Johnson unexpectedly capitulates to Washington.

John R. Bolton, Mr. Trump’s former national security adviser, has argued that the Americans can make this request themselves. The Europeans disagree, saying Mr. Trump’s repudiation of the nuclear accord last year means the United States is no longer a party to it.

Still, Ms. Geranmayeh said, both Iran and the United States are unpredictable. “If there is no diplomacy or something else poisons it militarily, the Europeans have triggered a clock that could end up more quickly at the Security Council,’’ she said.

The political unrest in Iran, following the killing of General Suleimani and Iran’s accidental shooting down of a civilian airliner, creates enormous uncertainty. The government in Iran is cracking down on protests, and with parliamentary elections next month, hard-line rhetoric is bound to increase.

While Iran insists its nuclear work will remain peaceful, its decisions to disregard the nuclear deal’s limits on both the volume and purity of Iranian nuclear fuel have raised worries that the country could amass enough enriched uranium to create a bomb in a matter of months.