President Emmanuel Macron of France and Chancellor Angela Merkel of Germany similarly argued that the Iran accord was flawed but could be built upon. The allies had suggested that the West could impose new sanctions against Iran’s ballistic missile development and armed support for the Syrian regime, for Hezbollah, for Hamas and Islamic Jihad in the Palestinian territories — but keep the nuclear deal intact.

“My view — I don’t know what your president will decide — is that he will get rid of this deal on his own, for domestic reasons,” Mr. Macron told a group of reporters in Washington during his visit last month.

But it has been difficult for Europeans to answer Mr. Trump’s demands because it is clear that Iran will not agree to reopening the terms of the accord itself. And if it were to be reopened, Iran had a series of demands of its own.

Should Mr. Trump withdraw from the accord, Iran could accurately claim that Washington was the first to violate it — a propaganda win. And Iran would be free, if it chose, to resume fuel production, according to diplomats who spoke on the condition of anonymity to discuss the sensitive negotiations.

The mantra of the European negotiators toiling to retain the nuclear deal has been “to fix it, not nix it.” But Mr. Trump has come at the problem from a different perspective: He argues that the only solution is a clean slate.

Mr. Trump has told visitors he believes that once the current agreement is destroyed, Iran will come to the table to negotiate a new one. Iran’s foreign minister, Mohammad Javad Zarif, who negotiated the initial accord with Mr. Kerry for more than two years, has said Tehran will not participate in such negotiations since it already spent years doing so.