WASHINGTON — The senior Senate Foreign Relations Committee Democrat listened as a firebrand Republican declared a bipartisan bill on Iran's nuclear program had "very little teeth." As the packed hearing room became quiet and tense, the Democrat did what his colleagues say he had done for over a week: He struck a conciliatory tone.

"I don't think we'll convince any administration, Democrat or Republican, that Congress should have any role in anything they do — we understand that," the Democratic senator quipped.

Many of the other 18 senators laughed, as did staffers and audience members. The Republican senator, Ron Johnson of Wisconsin, did not. But he also did not challenge the senior senator who is being credited by Republicans and Democrats alike with playing a major role in transforming a once-controversial bill into one the committee unanimously approved.

It was that veteran Democratic member, along with Foreign Relations Committee Chairman Sen. Bob Corker, R-Tenn., who chatted daily about sticking points in the negotiations last week with Sen. Chris Coons, D-Del., during his swing through Africa. And it was that Democrat who received the first across-the-aisle handshake from Corker after the chairman, with a relieved smile, gaveled closed the meeting following the 19-0 vote.

But that senior Democrat was not recently indicted Sen. Robert Menendez, D-N.J., the committee's embattled former chairman and ranking member. It was the man who only two weeks ago became its ranking Democrat, Sen. Ben Cardin of Maryland.

GSM2 Evan Peterson, from the USS Carney (DDG-64) demonstrates the new Improved Flame Resistant Variant flightsuit at Naval Weapons Station Yorktown in Yorktown, Va., on Thursday, September 10, 2015.
GSM2 Evan Peterson, from the USS Carney (DDG-64) demonstrates the new Improved Flame Resistant Variant flightsuit at Naval Weapons Station Yorktown in Yorktown, Va., on Thursday, September 10, 2015.

The serious-but-unassuming Cardin's legislative battlefield promotion thrust him into a political pressure cooker. And Republican and Democratic senators say he delivered in a big way.

The Foreign Relations Committee was readying a measure crafted by Corker and Menendez to set up a process under which both chambers would assess any deal the Obama administration and five other global powers broker with Iran over its nuclear program.

Administration officials for months had vigorously opposed it, even refusing to discuss specific gripes or fixes during meetings, according to Coons. But Corker and other Republicans, joined by many Senate Democrats, seemed buoyed by that opposition.

So with a high-stakes committee vote just days away, Cardin ascended to the position of top Democratic negotiator. What he pulled off in talks with Corker and other senators has won him several days of a ritual increasingly rare on Capitol Hill: bipartisan fêting.

"Cardin, look, he's easy to talk with. Easy to get on the phone. Had some issues that were red lines for him, of course," Corker told CongressWatch. "You know, he stepped right in and with a lot of extenuating circumstances and was a great partner. I look forward to working with him on other issues.

"There were a lot of people who made this effort very successful," Corker said. "But Ben Cardin has got the tone, the temperament, the personality, the accessibility, the reasonableness that it takes here in the Senate to solve big problems. It's a great pleasure to work with him."

Coons, in a brief interview, credited Cardin's "long experience as a legislator, in his state legislative experience before coming to the Congress, then in the House and now in the Senate.

"So he had a great sense of the art of the deal," Coons said, calling Corker and Cardin "tireless on this."

"I was in Africa last week, and literally spoke to each of them from Tunisia, Kenya and Senegal and Chad," Coons said. "I spoke to them almost every night for a week for half an hour."

GSM2 Evan Peterson, from the USS Carney (DDG-64) demonstrates the new Improved Flame Resistant Variant flightsuit at Naval Weapons Station Yorktown in Yorktown, Va., on Thursday, September 10, 2015.
GSM2 Evan Peterson, from the USS Carney (DDG-64) demonstrates the new Improved Flame Resistant Variant flightsuit at Naval Weapons Station Yorktown in Yorktown, Va., on Thursday, September 10, 2015.

During those conversations, Cardin sought "input on who else needed to have some conversations, where it needed to move, where it needed to be negotiated." And other senators, including GOP Sens. Marco Rubio of Florida and Cory Gardner of Colorado, reported to Coons they had similar experiences.

During the Tuesday afternoon markup, Rubio applauded Corker and Cardin for revising the original bill in a manner that allowed members of both parties — and even the White House — to sign on.

"I want to thank you, Mr. chairman and the ranking member, for your cooperation and help," Rubio told the duo. "We were able to work together."

Another Republican, Sen. Johnny Isakson of Georgia, also praised the panel's new ranking member.

"I just want to thank Chairman Corker and Ranking Member Cardin for their cooperation today," he said during the session. "I want to thank the chairman and the ranking member for everything they've done."

Valerie Jarrett, among the closest of President Barack Obama's White House advisers, on Wednesday said Cardin and Corker "did a terrific job of working together and listening, and in the end compromising."

Sen. Edward Markey, D-Mass., credited Cardin with doing "a very good job ... in mediating so that we could remove the provisions that were unrelated to the actual nuclear program in Iran."

"I think he is a very good negotiator. He went to the core of the issue and what was essential," Markey told CongressWatch. "He identified it. He worked with Bob Corker, and I think ultimately they both decided that was the proper course."

During the markup, Cardin explained to Corker his approach as ranking member as "working with you to achieve our mutual goals."

"And that is, this Senate Foreign Relations Committee has an extremely important role to play and we want to do that in the best interest of the United States," Cardin said. "So sure, I represent the Democratic members, but in a broader sense, I think we both represent all the members of the Senate in bringing as much unity as we possibly can to foreign policy in this country."

The Maryland Democrat also noted he senses "a strong, common commitment in the Congress of the United States and in the White House to make sure Iran never becomes a nuclear weapon state.

"That is our objective," he said sternly. "That's a game-changer for the Middle East. It's something we cannot allow to occur."

Himself a veteran legislator, Markey said that once Cardin joined the negotiations, he and Corker "worked hard to make sure it was something that didn't have subject matter that was unrelated and could, in fact, derail a consensus being formed."

Doing so required "tireless listening" and "being willing to compromise," Coons said.

With Menendez relegated to a support role as he battles federal corruption charges, Cardin was able to focus on crafting a truly bipartisan bill in an age of bitter partisan fighting and inaction.

Coons applauded Cardin for "believing this outcome was possible, even when so much of the current partisan Congress would suggest otherwise."

Twitter: @BennettJohnT