The South Carolina senator said Iran is "probably" in technical compliance with the accord, but claimed it is violating the spirit of the pact through its continued destabilizing activities in the Middle East, such as supporting terrorist groups. He said a renegotiated deal must be contingent on changing the regime's behavior.
"I think the biggest mistake in the deal, quite honestly, was disconnecting the behavior of the regime and sanctions relief," Graham said at a conference hosted by the Institute for the Study of War. "They can be in technical compliance with their nuclear program, but to give them a bunch of money and to not make them change their destabilizing behavior is a huge mistake. But I don't want to abandon the nuclear part of it unless we have a reason to because I know the world is invested in this deal."
Instead, Graham wants the administration to strengthen the accord by extending its time frame, particularly in regard to restrictions on the enrichment of uranium and production of plutonium, both of which are required to build a nuclear weapon. He also called for new limits on Tehran's ballistic missile program.
Persuading European allies to reenter negotiations, however, will prove difficult.
Envoys to the United States from the European Union, Britain, France, and Germany said on Monday the deal is working and declined to renegotiate its provisions. China and Russia have also stood adamantly opposed to reopening the agreement to negotiations.
Iran last week ruled out the possibility of revisiting the agreement. President Hassan Rouhani threatened to "respond decisively and resolutely" if the United States chooses to blow up the accord.
Under U.S. law, President Donald Trump must notify Congress every 90 days of whether Iran is adhering to the accord, which aimed to limit Tehran's nuclear activities in exchange for the lifting of sanctions related to the program. The Trump administration has twice recertified the agreement, though Trump warned in July he would not continue to do so indefinitely.
Trump, who called the deal an "embarrassment" before the United Nations, told reporters last week he had made up his mind on recertification. National Security Adviser H.R. McMaster remained tight-lipped on the decision when pressed by reporters Monday.
Secretary of State Rex Tillerson has signaled Trump is open to renegotiating the deal rather than abandoning it entirely. Tillerson last week took issue with the deal's sunset provision enabling Iran to eventually enrich uranium.
In the meantime, Graham said the United States needs to pursue additional sanctions against Iran separate from the nuclear accord. In July, Congress implemented a new round of sanctions on Iran targeting its ballistic missile program.