It is estimated that Iran could have enough nuclear fuel within weeks to produce a bomb, although manufacturing an actual warhead would take considerably longer. Iran has denied allegations that it is making a nuclear bomb and has maintained that its program is for peaceful purposes. The U.N. atomic watchdog, the International Atomic Energy Agency, has also said it has found no evidence that Iran is making a nuclear bomb.
Mr. Trump wanted to force Iran to negotiate a new agreement that would also hamper its missile program and its support for proxy militias across the Middle East that have stirred violence from Iraq to Syria to Yemen. Iran has steadfastly refused to discuss its missile and military activities or negotiate issues beyond a nuclear deal.
Biden administration officials have insisted that they, too, wanted to curb those programs, but diplomats said as recently as last month that they would not be part of any immediate agreement with Iran.
That has irritated a handful of Democrats who opposed the 2015 agreement, which was negotiated by the Obama administration.
“I ask why we would try to simply go back to the J.C.P.O.A. — a deal that was not sufficient in the first place and still doesn’t address some of the most serious national security concerns we have,” Senator Robert Menendez, Democrat of New Jersey and the chairman of the Foreign Relations Committee, said during a speech on Feb. 1. He was referring to the Joint Comprehensive Plan of Action, the formal name for the nuclear deal with Iran, which Britain, China, France, Germany and Russia also signed.
During his speech on the Senate floor, Mr. Menendez noted that Iran’s ballistic missile program is the largest in the region, and that it has been used to attack American troops in neighboring Iraq. Iran tried but failed to launch a satellite into orbit in December, and last week unveiled a new long-range missile that could reach Israel or other countries in the region. Yemeni rebels backed by Tehran have for years attacked Saudi Arabia and last month hit the United Arab Emirates with missile and drone strikes.
Mr. Menendez called Iran’s nuclear program a “clear and present danger” that has “grown disproportionately worse day by day.”
Even so, he said, “the time is now to reinvigorate our multilateral sanctions efforts and pursue new avenues, new ideas, new solutions for a diplomatic resolution.”
Other Democrats have urged the Biden administration to rejoin the nuclear deal as quickly as possible. Senator Christopher Murphy, Democrat of Connecticut, called it “laughable” that Republicans would claim that Mr. Trump’s sanctions policy had deterred Iran’s military activities.
“Trump tried it,” Mr. Murphy said in his own speech on the Senate floor on Feb. 8. “It didn’t work. Iran didn’t come to the table on anything.”
“News flash: Occasionally there are diplomatic agreements that are in the best interests of the United States, and the J.C.P.O.A. was inarguably one of them,” Mr. Murphy said.
The Biden administration does not have to seek congressional authority to recommit to the deal, although a 2015 law gives lawmakers the ability to review and potentially block it — a move that Mr. Biden would almost certainly veto.
Last week, 33 Republican senators warned in a letter to the White House that any deal would “likely be torn up” by the next presidential administration “as early as January 2025.” A letter signed by more than 100 House Republicans this week issued a similar threat.
The possible return of American sanctions in as soon as three years if a new deal is reached but again scrapped is a main reason Iran has been reluctant to commit to coming back into compliance. Leaders in Tehran want assurances that the agreement will endure under future presidents — which the Biden administration cannot promise.
“At the very least their parliaments, or heads of parliaments, including Congress in America, should issue a political statement announcing their support of the agreement and a return to J.C.P.O.A.,” Iran’s foreign minister, Hossein Amir Abdollahian, told the Financial Times in an interview published Wednesday.
Even if a new agreement lasts three years, American diplomats and other supporters said it would still meet its main objectives: easing Iran’s economic pain while slowing its suspected march to a nuclear bomb.
Jeremy Ben-Ami, the president of the liberal pro-Israel advocacy group J Street, described the opposition to the accord as a small number of Democrats and a concentrated Republican effort “to play politics with American foreign policy.” He said no Democrats who voted to support the 2015 deal lost elections in congressional midterm elections a year later.
“There was no political fallout,” said Mr. Ben-Ami, whose group supported the agreement and is advocating its renewal.
“The beauty of having this argument a second time is that we actually have the facts from the first time — the real-world experience both that the policy was good and that the politics didn’t hurt anybody who supported it,” Mr. Ben-Ami said. “And yet, here we are in 2022, going back and having the exact same argument.”