A Republican senator who once had to reassure voters he didn’t think he was “Rambo” and was a mixed martial arts fighter before entering politics got into a vocal brawl with a union boss during a public congressional hearing, saying: “You need to shut your mouth.”

Markwayne Mullin of Oklahoma exchanged verbal fire with Sean O’Brien, president of the Teamsters, during a hearing staged on Wednesday by the Senate health, education, labor and pensions committee.

The chair, the Vermont independent Bernie Sanders, was seeking support for his Protecting the Right to Organise Act. But Mullin made headlines of his own.

The 45-year-old, who owns a plumbing business, said he was “not against unions …some of my very good friends work for unions. They work hard, and they do a good job.”

But he said he did not like “intimidation” by union leaders trying to unionise businesses including his own.

“I’m not afraid of a physical confrontation,” Mullin continued. “In fact, sometimes I look forward to it. That’s not my problem.”

In late 2021, Mullin memorably said “I’m not Rambo”, in reference to a character played by Sylvester Stallone in a violent film series, amid controversy over an attempt to enter Afghanistan with a private security team. He also said he had not tried to be “a cowboy or anything like that”.

Mullin is a state wrestling hall of fame member whose website says he is “a former Mixed Martial Arts fighter with a professional record of 5-0”.

Addressing O’Brien, he said: “But when you’re [confronting] my employees? For what? Because we were paying higher wages? Because we had better benefits and we wasn’t requiring them to pay your guys’ exorbitant salaries?”

The website of the International Brotherhood of Teamsters describes O’Brien, 51, as a fourth-generation teamster who started out in “the rigging industry as a heavy-equipment driver in the Greater Boston area”.

Mullin asked O’Brien about his salary and accused him of forcing members to pay union dues.

“You’re out of line,” O’Brien said.

“Don’t tell me I’m out of line,” Mullin said. “You need to shut your mouth.”

O’Brien mocked Mullin’s “tough guy” act.

Sanders tried to gavel the two men to order, saying: “Senator, hold it, hold it.”

O’Brien told Mullin: “I bet you I work more hours than you do. Twice as many hours.”

Mullin said: “Sir, you don’t know what hard work is.”

O’Brien said unions “create opportunity because we hold … greedy CEOs like yourself accountable”.

Mullin said: “You calling me a greedy CEO?”

O’Brien said: “Oh yeah, you are. You want to attack my salary, I’ll attack yours … What did you make when you owned your company?”

Mullin said he made “about $50,000 a year because I invested every penny”.

“OK, all right,” O’Brien said. “You mean you hid money?”

Pointing at O’Brien, Mullin said: “Hold on a second.”

“All right, we’re even,” said O’Brien, smiling. “We’re even.”

Mullin said: “We’re not even. We’re not even close to being even. You think you’re smart? You think you’re funny?”

“You think you’re funny,” O’Brien said. “You framed your opening statement saying you’re a tough guy.”

Sanders said: “Senator, please continue your statement.”

Mullin said: “I think it’s great you’re doing this because this shows their behavior and how they try to come in and organise a shop.”

Sanders said: “They see your behavior here. Stay on the issue.”

After the hearing, the spat continued on social media.