“I suspect that this will be a very difficult conversation,” said Wendy R. Sherman, the former top State Department official who negotiated the Iran deal for Mr. Obama. “I’m sure that Macron will say how important staying in the deal is to a strong trans-Atlantic relationship in all things, particularly security. I think Merkel will deliver the same message on Friday.”
Even so, the White House signaled Monday that Mr. Trump enters the talks with one set impression: “He thinks it’s a bad deal — that certainly has not changed,” said Sarah Huckabee Sanders, the White House press secretary.
If Mr. Macron and Ms. Merkel can persuade Mr. Trump to stick by the Iran agreement for now, it could influence the president’s forthcoming meeting with Kim Jong-un, the North Korean leader who already has a small nuclear arsenal. Whatever its flaws, American officials understand that canceling the Iran deal days or weeks before that meeting might complicate Mr. Trump’s chances of making an agreement with Mr. Kim.
Mohammad Javad Zarif, Iran’s foreign minister, implicitly made that point on Monday by noting that the negotiations that led to the nuclear agreement between his country and six world powers involved give and take by all sides.
“And now the United States is saying, ‘What’s mine is mine and what’s yours is negotiable. But whatever I gave you, now I want it back,’” Mr. Zarif said in an interview with The National Interest, a Washington policy magazine. “Who would, in their right mind, deal with the U.S. anymore?”
Mr. Trump faces conflicting positions among his own advisers as he reconstitutes his national security team. John R. Bolton, his new national security adviser, has long advocated simply ending the Iran deal, while Mike Pompeo, set to become secretary of state, is open to keeping it if strong new provisions can be negotiated.
Mr. Macron arrived in Washington to a festive welcome: American and French flags flew on Pennsylvania Avenue as he and his wife, Brigitte Macron, arrived at the White House and were greeted by Mr. Trump and the first lady, Melania Trump.
Mr. Macron reached in for a hug and kissed Mr. Trump on both cheeks, French-style, a sign of their warm ties. The two couples headed inside for a few minutes and then out to the South Lawn, smiling and chatting casually as cameras recorded the moment.
Wielding shovels, the two presidents moved some dirt around where a tree was to be planted, a gift from the Macrons. The tree, a European sessile oak, came from Belleau Wood, where, during World War I, nearly 10,000 American Marines were killed or injured in battle in June 1918. From there, the two couples flew by helicopter to George Washington’s Mount Vernon estate for dinner.
The Macrons will return to the White House on Tuesday morning for a pomp-filled arrival ceremony on the South Lawn, complete with members of all five branches of the military in formal uniforms. The two presidents will hold meetings and conduct a joint news conference. In the evening, the Trumps will host their first state dinner, featuring rack of spring lamb and Carolina gold rice jambalaya cooked New Orleans style.
Mr. Trump, 71, and Mr. Macron, 40, have forged an unlikely friendship, despite their political differences over the Iran deal, international trade, climate change and other issues. But while Europeans consider Mr. Macron their envoy to Mr. Trump, he has had mixed success influencing the president. The two leaders teamed up to launch airstrikes against Syria this month in retaliation for a suspected chemical attack, but when Mr. Macron publicly said he had persuaded Mr. Trump to keep American troops in the country “for the long term,” the White House quickly rebutted him.
Mr. Macron has stronger credentials on the Iran deal than Britain or Germany because France was the most fervent in pressing for tougher provisions in 2015. Having learned the best way to communicate with Mr. Trump, Mr. Macron telegraphed his message by appearing on the president’s favorite network, Fox News, over the weekend.
Is the pact “a perfect thing for our relationship with Iran? No,” Mr. Macron said on “Fox News Sunday.” “But for nuclear, what do you have as a better option? I don’t see it. What is the what-if scenario or your Plan B? I don’t have any Plan B for nuclear against Iran.”
Mr. Macron added that he was “not satisfied” with the situation in Iran and supported modifications to the deal, known as the Joint Comprehensive Plan of Action, or J.C.P.O.A. “My point is to say, don’t leave now to J.C.P.O.A. as long as you don’t have a better option for nuclear, and let’s complete it with ballistic missile and a regional containment,” Mr. Macron said.
Mr. Zarif picked up on that on Monday to warn that there would be no substitute for the nuclear accord if Mr. Trump withdrew.
“President Macron is correct in saying there’s no ‘Plan B’ on JCPOA,” Mr. Zarif wrote on Twitter. “It’s either all or nothing. European leaders should encourage President Trump not just to stay in the nuclear deal, but more importantly to begin implementing his part of the bargain in good faith.”
The negotiations with European officials, led by Brian Hook, the State Department’s director of policy planning, have found some common ground, according to people briefed on the talks.
Negotiators have agreed on measures to impose sanctions on Iran if it tests long-range missiles, but are still divided on how to respond to testing of short- and medium-range missiles. They have agreed that Iran would be sanctioned if it blocks international nuclear inspectors from military sites. And they have made progress in defining a trigger for reimposing sanctions — if Iran were found to have expanded its nuclear program enough to allow it to develop a weapon in less than a year.
But negotiators are divided over what would happen then. The Trump administration wants sanctions to be imposed automatically if Iran trips that wire, while the Europeans want a reassessment to determine whether the expansion is consistent with Iran’s civilian nuclear program.
“The Europeans have moved very far in a few months, and I think this should be bridgeable, but of course it really depends on Macron and Trump,” said Mark Dubowitz, the chief executive of the Foundation for Defense of Democracies. “It sounds narrow, but it’s actually pretty fundamental. It’s entirely possible that the thing breaks down on that basis.”
Russia and China, the other parties to the original deal, have resisted any changes. “We will obstruct attempts to sabotage these agreements, which were enshrined in a U.N. Security Council resolution,” Sergey V. Lavrov, the Russian foreign minister, said Monday after meeting with his Chinese counterpart, Wang Yi, in Beijing.
But critics of the deal pressed Mr. Trump to remain strong. On Monday in Jerusalem, Prime Minister Benjamin Netanyahu of Israel again condemned the agreement.
“Israel will not allow regimes that seek our annihilation to acquire nuclear weapons,” he said.
“This is why we opposed so resolutely the Iran deal — because it gives Iran a clear path to a nuclear arsenal” and “does not deal with the ballistic missiles that can deliver this weapon to many, many countries,” he added. “This is why this deal has to be either fully fixed or fully nixed.”