At nearly every step in his career Menendez has broken barriers for Hispanics, including when he replaced Kerry as the Foreign Relations chairman. (By seniority, Democratic Sen. Barbara Boxer of California was next in line, but she chose to remain chair of the Environment and Public Works Committee instead.)

Menendez had little time to celebrate his latest ascent, though.

Days before he became chairman, FBI agents raided the South Florida medical offices of a prominent Menendez donor, eye doctor Salomon Melgen. The fallout enveloped Menendez in an ethical cloud and raised questions about free flights he took on Melgen’s private plane, political aid on behalf of Melgen and shadowy allegations about Menendez taking trips to visit prostitutes in the Dominican Republic, advanced by an anonymous e-mailer and the conservative Web site the Daily Caller.

The story went national. At one point, reporters chased Menendez down a hallway at the Marriott Wardman Park in Washington, shouting questions about Dominican hookers as the senator sped toward an elevator. Citing the ties to Melgen, the New York Times editorial page called on Menendez to step down as foreign relations chairman. It derided him as “never a distinguished choice” to lead the committee.

The salacious allegations involving prostitutes, however, crumbled under independent scrutiny (some accusers said they were paid to make up their statements), while the flap over the flights has faded away, at least for now. (Menendez repaid Melgen $58,500 for the two round-trips.)

When Menendez emerged from his defensive crouch, he gave the first major speech of his chairmanship at an AIPAC conference in Washington. “There will never be any daylight between the United States and Israel on my watch. Never,” he declared. “Not on my watch.”

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People who have worked with and against Menendez describe him as sharp, intensely prepared and demanding. Friends say he has a comic side in private, but it’s rarely seen in public, where he bristles with probing questions.

“He’s a blunt guy—sometimes blunt to a fault—but you don’t have to question where he stands,” says Frank Sharry, founder of America’s Voice, a national immigration advocacy group, when I spoke to him about immigration reform earlier this year.

“He lets you know exactly where he stands, and that is vital if you’re going to have serious negotiations,” says McCain, who worked with Menendez when the two were pushing the “Gang of Eight” Senate immigration bill.

That bill showed Menendez’s less-heralded practical side: He accepted a more arduous pathway to citizenship than he would have preferred, as well as a “border surge” intended to win over wavering Republicans, but he stood firm on his most prized principles, insisting that the final bill include some route to legalized status and preserve an emphasis on family reunification for undocumented immigrants.

A sense of pragmatism remains in the sanctions bill, according to several foreign affairs analysts. Menendez’s plan includes a six-month window for negotiations and up to a year with potential extensions. That running room, says Johns Hopkins University’s Charles Stevenson, gives the White House time to work while allowing lawmakers to express their skepticism and satisfy their own constituents. “Making clear to the Iranians that President Obama doesn’t just have to satisfy them, he has to satisfy his domestic constituencies, strengthens his hand,” argues Georgetown University’s Matthew Kroenig, a former special adviser in the Office of Secretary of Defense.

Menendez says the threat of sanctions will let the Iranians know that a hammer is poised to strike if they are simply stalling. “If this all falls apart, we don’t have months,” he told me.

Obama rejected that idea Friday. “It’s not going to be hard for us to turn the dials back or strengthen sanctions even further,” he said. “I'll work with members of Congress to put even more pressure on Iran, but there’s no reason to do it right now.”

Menendez’s plan faces a steep climb. Senate Majority Leader Harry Reid (D-Nev.) will determine if or when it gets a vote when Congress returns in January, and Menendez is facing pushback from fellow Democrats. Ten committee chairs wrote Reid this week urging him to keep the Senate from unilaterally advancing new sanctions and potentially scuttling negotiations.

Menendez is not backing down. In a recent hearing, he reminded two administration officials—the same ones he had scolded in 2011—how they argued against him then, only to see his bill clear the Senate 100-0.

But in our interview two days earlier, Menendez insisted he doesn’t hold grudges.

“I don’t live my life in a rearview mirror,” he said. “If I did, by God, there would be so much stuff going on.”

Jonathan Tamari is the Washington correspondent for the Philadelphia Inquirer.