“He wants to do many things,” Iran’s president, Hassan Rouhani, said of Mr. Trump in a speech. During the campaign, Mr. Trump called the nuclear agreement “a bad deal” that he promised to “tear up.”
“He wants to undermine the J.C.P.O.A.,” Mr. Rouhani said, referring to the agreement, the Joint Comprehensive Plan of Action. “He wants to tear up the J.C.P.O.A. Will we or our nation allow this?”
As Mr. Rouhani posed the question, the audience began chanting, “Death to America.”
In the speech, which was delivered at Tehran University and televised nationally, Mr. Rouhani maintained that “America cannot influence our determination, this nation’s resistance and its struggle.”
He continued: “America is our enemy; we have no doubt about this. The Americans want to put as much pressure on us as they can.”
The change in tone by the Iranian president comes as he faces increasing pressure from hard-liners, who argue that the United States is not to be trusted to keep its end of the deal. They say that a recent congressional vote to extend unilateral American sanctions against Iran confirmed their warnings and that the nuclear deal is a failure.
For Mr. Rouhani, who until recently seemed set to be elected for a second term in May, the change in fortune is a threat to his political career, analysts say.
“Trump is far worse than his predecessor,” said Hamidreza Taraghi, a hard-line political analyst. “Rouhani has proven that trusting America is useless and a waste of time, energy and money. He should not be re-elected.”
Mr. Rouhani had promised an economic renaissance after the 2015 deal, which led to the lifting of most of the sanctions that had been crippling the Iranian economy in exchange for the mothballing of Iran’s nuclear program.
But that has not materialized, thanks to remaining sanctions over Iran’s ballistic missile program and terrorism, as well as lingering fears among Western companies about incurring American penalties for doing business with Iran.
During his speech on Tuesday, Mr. Rouhani, apparently trying to fend off the attacks, emphasized that ultimately it was the nation’s supreme leader, Ayatollah Ali Khamenei, who had led the negotiations and signed off on the deal.
“We took no step in the issue of the nuclear agreement without consulting the supreme leader,” he said.
Ayatollah Khamenei has often criticized the deal publicly, but analysts agree that without his specific consent, a compromise with the United States and other global powers would never have been reached.
Mr. Trump’s ascent to power has been met by Mr. Rouhani and other Iranian leaders with familiar slogans like “America can’t do a damn thing.” But the vote last week in Washington to extend sanctions for another decade, which they say violated the nuclear agreement, seemed to strike a nerve.
“If the Iran Sanctions Act is carried out, it will be a clear and obvious violation of the agreement and will be met with a very harsh response from us,” Mr. Rouhani said, joining dozens of other clerics, politicians and generals who have made similar threats.
He told Iranians not to be “concerned,” saying that a government committee would meet on Wednesday to determine Iran’s reaction to the extension of sanctions, which the Obama administration said did not violate the nuclear agreement.
Iranian lawmakers have proposed a boycott of American products, but very few are being sold in the country. Some have even suggested restarting nuclear activities and the enrichment of uranium, though they have little or no say over such matters.
Pulling out of the deal is not an option for Iran, a Foreign Ministry spokesman said on Monday. “Iran will not be the first to break the agreement,” Bahram Ghasemi, the spokesman, said. “We will never, and in no way will be the first to make the first step forward.”