Menendez, who vocally opposed the nuclear deal, joined Schumer and Cardin in voting against it in 2015. With Trump threatening to pull out if Congress fails to pass legislation to address what he sees as flaws in the deal, advocates are worried that Menendez might champion measures that prove unacceptable to Tehran, triggering the agreement’s collapse.

Senate Minority Leader Chuck Schumer, D-N.Y., allowed Menendez to resume his post on Feb. 6, after the Department of Justice dropped corruption charges against him. Sen. Ben Cardin, D-Md., had been filling in since April 2015 while Menendez remained under a cloud of suspicion and stood trial.

“I think the president would be better served getting our European allies to join us in our multilateral sanctions effort,” Menendez said. He called on the White House to build international support for revisiting the deal’s so-called sunset clauses, under which nuclear and other restrictions on Iran lessen over time.

In a hallway interview, however, Sen. Bob Menendez, D-N.J., told Al-Monitor that he does not plan to use his perch as the top Democrat on the Senate Foreign Relations Committee to try to rip up the agreement. Instead, Menendez expressed skepticism about President Donald Trump’s threats to withdraw from the agreement, even as he himself champions further sanctions against Tehran for its non-nuclear activities.

Defenders of the 2015 Iranian nuclear deal responded with trepidation to news this week that the Democrats’ staunchest Iran hawk would be reclaiming a top congressional role.

“We are very concerned that Sen. Menendez is going to help Donald Trump tear up the Iran deal,” Jamal Abdi, vice president for policy at the pro-deal National Iranian American Council, told Al-Monitor. “The onus is really on him to demonstrate his willingness to stand up to Trump on Iran, because right now there is not a lot of confidence given his past efforts.”

For now, Menendez’s insistence on a multilateral approach with Europe seems to square with Cardin’s own efforts. That has left the deal’s proponents feeling cautiously optimistic.

“Cardin … was also among the small handful of Democrats who did not vote to support the deal, but he did fight back against Republican-led efforts to unravel it,” said Abdi. “Menendez is someone who voted against the Iraq War [in 2003], so hopefully he understands the gravity of what Trump appears to be aiming for with Iran and will work to protect the nuclear deal and prevent a war.”

Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., is working with the White House on legislation that would reinstate sanctions on Iran if it comes within one year of obtaining a nuclear weapon. Cardin has adamantly insisted, however, that any such efforts not violate the nuclear accord and that they be conducted in concert with the United States’ European allies, who are wary of running afoul of the deal.

Although Menendez had not previously weighed in on Trump’s threats to withdraw from the agreement, he has criticized the president’s October decision not to certify that the deal meets congressional requirements.

“Refusing to certify Iran’s compliance with a deeply flawed deal … does little to address its shortcomings,” Menendez said on the Senate floor last month. “The president’s move … was a hollow attempt to fulfill a campaign promise while continuing to let Iran enjoy the benefits of [the deal] and continue its non-nuclear malign activities throughout the region. Furthermore, this decision was carried out without a clear path forward and without specific proposals that could have garnered international support.”

Trump’s October decision gave Republicans the opportunity to reinstate expansive sanctions with a simple majority of just 50 votes instead of the usual 60, but the White House and Congress opted instead to try to amend existing law in an effort to address the deal’s sunset provisions, a move that requires getting at least nine Democrats on board since Republicans only have a 51-49 majority.

While Menendez may not be willing to help Trump unravel the nuclear deal, he has, however, joined other hawkish Democrats in complaining that the president has not used the full extent of his authority to implement non-nuclear sanctions against Iran. Congress last year passed the first new set of sanctions on Iran since the deal was signed in 2015.

“The sanctions … include provisions on human rights violations and so the administration can use those provisions as it relates to any entity that is either engaged in assisting, or actually conducting, human rights violations on Iranians,” Menendez told Al-Monitor in January. “So if you really want to be tough on Iran outside of its nuclear proliferation, you have plenty of things you can do. And you have the arms to do it already, you just have to go ahead and pull the trigger.”

Republicans are also lobbying the administration to place additional sanctions on Iran. Sen. Marco Rubio, R-Fla., who in December urged Trump to “consider” reinstating nuclear sanctions on Iran, this month led 13 other Republican senators in a letter asking the president for more sanctions in regard to Iran's ballistic missile program.

In the House, Foreign Affairs Committee Chairman Ed Royce, R-Calif., penned a letter to Treasury Secretary Steve Mnuchin last month asking the administration to use its influence to ensure that the Paris-based Financial Action Task Force resume international anti-money-laundering efforts against Iran.