Trump faces an Oct. 15 deadline to say whether Iran is complying with terms of the deal and whether he considers the agreement to be in the U.S. national interest. His administration has recently signaled that he is likely to say no, raising the specter of renewed U.S. sanctions and the possibility that the deal would fall apart.
"You'll see what I'm going to be doing very shortly in October," Trump told reporters Thursday when asked about his decision. "The Iran deal is one of the worst deals I've ever seen. Certainly, at a minimum, the spirit of the deal is just atrociously kept."
The president added that "the Iran deal is not a fair deal to this country. It's a deal that should have never, ever been made. . . . We are not going to stand for what they are doing with our country. They've violated so many different elements, but they've also violated the spirit of that deal."
Most who will be in the audience for Trump's speech disagree. The European Union, one of the architects of the deal, hopes to hold a meeting of the signatories, including Iran, on the sidelines of the General Assembly session.
"This agreement is a very important agreement," U.N. Secretary General António Guterres said Wednesday. "It contributed to an important de-escalation at the moment, and it is a factor of stability. And it's my opinion that all parties should do everything possible for this agreement to be preserved."
White House officials sketched out an ambitious series of events for Trump, including bilateral meetings Monday with French President Emmanuel Macron and Israeli Prime Minister Benjamin Netanyahu.
Trump is expected to focus on "Iran's destabilizing behavior, including its violation of the sovereignty of nations across the Middle East," national security adviser H.R. McMaster told reporters Friday.
The president also will have lunch with Guterres, and he'll meet with leaders of Jordan, the Palestinian Authority, the United Kingdom, Egypt, Turkey, Afghanistan and Ukraine. And Trump will hold a trilateral dinner with Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae-in to discuss North Korea's nuclear weapons threat.
While Trump's debut on Tuesday is perhaps the most highly anticipated moment, the U.N. gathering is also notable for who will not be there — Russian President Vladimir Putin and China's Xi Jinping, who are skipping this year's meeting. Aung San Suu Kyi, the de facto head of Burma, also will not attend amid a spate of government-backed ethnic violence in that country that has drawn international condemnation.
Trump has been a skeptic of international organizations such as the United Nations and NATO. He pulled the United States out of the Paris climate agreement and an Asia-Pacific trade accord, promoting a foreign policy aimed at limiting U.S. interventionism abroad in favor of domestic priorities.
Yet Trump administration officials said the president and his team are intent on having a strong presence and demonstrating leadership in New York on issues including terrorism, trade and human rights. Vice President Pence and Secretary of State Rex Tillerson will join Trump in the U.S. delegation.
"No one is going to grip-and-grin," U.N. Ambassador Nikki Haley said of handshake photo-ops. "The United States is going to work. . . . This is a time to be serious, and it's a time for us to talk out these challenges and make sure there's action that follows it."
As he did with NATO, Trump has pressed the United Nations for reforms, and Haley emphasized that the administration has seen improvements. She said the world body has moved away from "focusing on the commas and the periods" of toothless resolutions and begun taking stronger actions.
She cited recent decisions by the global body to enact sweeping economic sanctions on North Korea over its nuclear and ballistic weapons program.
Haley said the sanctions will hurt Pyongyang, but quickly added that if North Korean leader Kim Jong Un does not end his provocative missile and nuclear tests, the United Nations will run out of options.
"Having said that, I have no problem kicking it to [Defense Secretary James] Mattis because I think he has plenty of options," she said, alluding to military power.
The administration has options on Iran, too, which has colored Trump's engagement with U.S. allies and partners since his surprise victory in November.
Champions of the Iran nuclear deal were reassured when Trump twice acted to certify Iranian compliance. Those notifications, required by Congress, were taken as a sign that Trump might complain about the deal but would not "rip it up" as he had pledged to do as a candidate.
Now administration officials are telling allies they want to strengthen the deal, at the least.
Haley recently said the deal cannot be "too big to fail," no matter the heavy investment of important U.S. allies in keeping it intact. And Tillerson said Thursday that Iran is "clearly in default" of expectations and responsibilities under the deal, an assertion that Iran and European allies dispute.
The U.N. nuclear watchdog, the International Atomic Energy Agency, said this month that Iran is playing by the rules.
Trump is expected to cite Iranian ballistic missile activity and alleged Iranian support for terrorism and other activities as evidence that Iran is violating the "spirit" of the deal.
"President Trump has made it clear to those of us who are helping him develop this policy that we must take into account the totality of Iranian threats, not just Iran's nuclear capabilities," Tillerson said Thursday.
He quoted from the preface to the deal, formally known as the Joint Comprehensive Plan of Action, or JCPOA. The preface says that signers "anticipate that full implementation of this JCPOA will positively contribute to regional and international peace and security."
The Trump administration is arguing that those words convey obligations to Iran to curtail activities that would harm international peace and security.
"In our view, Iran is clearly in default of these expectations of the JCPOA through their actions to prop up the Assad regime, to engage in malicious activities in the region, including cyber activity, aggressively developing ballistic missiles," Tillerson said.
On Twitter, Iranian Foreign Minister Mohammad Javad Zarif said the deal is not renegotiable and said a "better" deal is "pure fantasy."
"About time for U.S. to stop spinning and begin complying, just like Iran," Zarif wrote.