WASHINGTON - Secretary of State John Kerry came to Capitol Hill twice last week to urge lawmakers not to advance any new sanctions on Iran, warning that they could disrupt delicate negotiations that have the United States and other world powers working on a potentially historic deal to stall the regime's nuclear program.

On Friday, though, Sen. Robert Menendez (D., N.J.) said he was still pursuing the idea.

That possibility, with implications on national and international debates, provides a glimpse of the strong-willed style that has long defined Menendez, 59, and that he has displayed in his first year as chairman of the Senate Foreign Relations Committee.

"He believes that Congress has an independent duty and Bob has stepped up to the plate," said Sen. Lindsey Graham (R., S.C.), who has collaborated with Menendez on several issues and also serves on the committee. "He's been a really active, bipartisan chairman."

The latest events have presented Menendez with his most visible early test as chairman, coming at the end of a tumultuous year that began with scandal and ended with his playing a prominent role on the international stage.

He has sharply questioned the short-term agreement that the United States and five other world powers have struck with Iran, and has urged lawmakers to consider new sanctions, set to take effect at the end of a six-month interim agreement if no final deal is reached.

"I'm getting nervous about what I perceive will be acceptable to [the administration] as the final status," he told two top Obama administration officials at a hearing Thursday.

Sen. Bob Casey (D., Pa.), who served on the committee until this summer, said Menendez has led the panel "with a measure of independence that I think every chairman should have."

While some senators see it as an increased role for Congress, others view Menendez's posture as a threat to talks that might peacefully end Iran's nuclear-weapons program. The six-month pact struck last month calls for modest relief for Iran and no new penalties as the Iranians, Americans, and U.S. partners work toward a long-term accord.

"If there is any chance at all that new sanctions at all might disrupt the agreement . . . why on earth would we risk that?" Sen. Jay Rockefeller (D., W.Va.) asked in a speech on the Senate floor.

A liberal on domestic issues but a hawk on the Middle East, Menendez has also bucked the president on a push to ease relations with Cuba. Menendez's parents left the country and came to New Jersey a few years before Fidel Castro took power.

In many ways, Menendez is a sharp change from Kerry, who led the foreign relations panel before him and was a faithful Obama ally.

But Menendez, who has long supported Israel and targeted Iran's nuclear program, dismissed any suggestions of a rift with Obama.

"In the midst of any moment of maybe difference of opinion, people lose sight of the overwhelming support that this committee has given and that I have given to the administration," he said last week.

He pointed, for example, to his support during hearings on the attacks on the U.S. Consulate in Benghazi, Libya, and his recent work to win Senate approval of an international treaty on disabilities.

Most notably, when Obama proposed military strikes in Syria, Menendez pushed a supportive bill through his committee and strongly backed the president even as many members of Congress criticized the idea. Obama, Vice President Biden, and Kerry all called to thank him, a Menendez aide said.

"We have an honest difference of opinion about how do we achieve our mutual end goal," on Iran, Menendez said.

By Friday, however, his push for new, provisional sanctions appeared to be losing steam.

"The White House has beat down people on the Democratic side," said Sen. Bob Corker of Tennessee, the top Republican on the Foreign Relations Committee. "It's not going to happen."

Menendez said he still hopes to roll out the sanctions, a Senate resolution, or some combination this week. He argues that the threat of new penalties will force Iran to negotiate in good faith for a long-term agreement.

Even if his effort stalls, the fact that Menendez is playing a role in a massive international debate itself is a stunning personal turnaround at the end of a year that began under a cloud.

He easily won reelection last November, and began 2013 poised to become chairman of foreign relations. But days before he was to take that latest step in a fast-rising career, news broke of an FBI raid on a Menendez campaign donor's medical office in Florida.

Menendez had taken free flights on the donor's private plane and there were unsubstantiated allegations, advanced by an anonymous e-mailer and conservative website, about trysts with Dominican prostitutes. The questions went national.

Suddenly, Menendez was facing calls from the New York Times editorial board to step down as foreign relations chair.

But the scandal faded: The prostitution allegations fell apart (some accusers said they were paid to make up stories). Questions about the flights (which Menendez repaid) remain in the hands of the Senate ethics committee.

He went on to help muscle immigration reform, a longtime priority, through the Senate - striking compromises to win Republican support, but standing firm on key issues such as a pathway to citizenship.

He played a key role in the national debate on Syria, and now on Iran. And last week, he proposed to fellow New Jerseyan Alicia Mucci in the Capitol Rotunda. She accepted.

For once, a proposal in the Capitol went smoothly. The sparring on Iran is set to resume this week.