As chairman of the Foreign Relations Committee, Menendez has expressed deep concerns about a six-month deal that the Obama administration and U.S. allies hope will lead to a long-term agreement that peacefully ends the threat of Iran's obtaining nuclear bombs.
Iran's initial concessions on its nuclear program, Menendez has said, do not match the short-term relief it is receiving.
"It seems like we've left the Iranians largely running in place - but they're running," Menendez said Tuesday.
While many lawmakers hold similar views, Menendez's role as committee chairman, a post he assumed Feb. 1, has given him a lead role in the debate and on the world stage - to the delight of like-minded Senate allies but to the concern of the Obama administration, which has blanched at Congress' appetite for imposing new penalties on Iran even as talks continue.
"This is a very delicate diplomatic moment," Secretary of State John Kerry told the House Foreign Affairs Committee on Tuesday. "We're at one of those really hinge points in history."
One outcome, Kerry said, could be a lasting resolution to Iran's nuclear program. The other could be more conflict. Adding sanctions now, he argued, could lead to the worse result, either by giving Iran an excuse to back out of talks or by angering U.S. allies who have agreed not to impose new sanctions while they work toward a long-term agreement.
"We have an obligation to give these negotiations an opportunity to succeed," Kerry said, arguing that sanctions can be quickly added if talks fail.
Independent analysts and scholarly journals such as Foreign Policy magazine have painted Menendez as a potentially destructive force to what could be a historic deal.
"If the United States goes to war with Iran," a Foreign Policy post on Facebook was headlined, "blame this New Jersey Democrat." A New York Times editorial Tuesday ripped Menendez for continuing to push sanctions.
Menendez said he was used to being pounded by Times editorials. "You mean number four?" he scoffed.
The senator, a product of Hudson County, some of New Jersey's most combative political turf, is a longtime ally of Israel and has been a strong voice in the debate over Iran for years - going back more than a decade on the House Foreign Affairs Committee.
But his views took on added weight when he became chairman of the Senate foreign relations panel early in his second term in that chamber.
The Iran debate is perhaps the most consequential first-year test of his tenure - and his display of an independent streak has won praise from several foreign policy hawks.
"He has exercised, in my view, a very dynamic leadership," said Sen. John McCain (R., Ariz.), who has sided with Menendez on Iran and in an earlier debate on Syria. "That's what a lot of us who have had an opportunity to chair a committee have also done: We are a coequal branch of the government."
As the administration's talks with Iran continue, Menendez said, the threat of new sanctions could provide more leverage for Obama and ensure that the regime there is not simply stalling.
The senator said his plan "will give the administration the flexibility they want for the purposes of their negotiations, but will also poise us to be ready to pursue additional sanctions should the Iranians either violate the framework agreement, or should they not come to a final agreement."
If talks falter, he said, Iran could quickly resume its march toward a nuclear bomb - so sanctions cannot be delayed.
"If this all falls apart, we don't have months," he said. "I think [sanctions] strengthened the administration's hand."
Indeed, Kerry - who preceded Menendez as Senate foreign relations chairman - credited previous sanctions with forcing Iran to the negotiating table. But he said the point was to bring about the serious negotiations now underway.
Menendez happily pointed out that the latest round of those sanctions were passed at his urging, over stiff objections from the administration.
Still, he stressed that he and the White House are in accord on most foreign policy issues, citing Syria and an international treaty on disabilities as two recent examples. But his defiance on Iran comes at a high-profile moment.
Independent analysts said it's not unusual for foreign relations chairmen to criticize presidents of the same party.
Charles Stevenson, who for 22 years was a Senate aide on foreign affairs, said Menendez and other lawmakers are maintaining a tough (and politically popular) stance while also giving Obama's team room to work.
"It still seems to me that Menendez is trying to be accommodating to the administration while still maintaining a skeptical position," said Stevenson, a lecturer at the Johns Hopkins School of Advanced International Studies.
And while Menendez and others, including Sen. Robert P. Casey Jr. (D., Pa.), have pushed for new sanctions, it's not clear when they might pass. With the Senate set to conclude its 2013 work next week, there almost certainly won't be time to vote by Dec. 31, Menendez conceded.
It's not clear if Majority Leader Harry Reid (D., Nev.), who controls the Senate agenda and is a close Obama ally, will allow a vote early next year.
Matthew Kroenig, a professor at Georgetown University and former adviser on Iran for the secretary of defense, said lawmakers such as Menendez are trying to thread a needle between aggressive action and not disrupting the talks.
The threat of sanctions, Kroenig said, may help the administration despite its objections.
"It's a constant reminder to the Iranians that we do have other options," he said, and signals Tehran "that President Obama doesn't just have to satisfy them, he has to satisfy his domestic constituencies."
Including one Democrat from New Jersey.