Vermont Sen. Bernie Sanders, the chairman of the Senate panel holding the hearing, yelled at Mullin to sit down after he challenged Sean O’Brien to a fight. Mullin had stood up from his seat at the dais and appeared to start taking his ring off.

“This is the time, this is the place,” Mullin told O’Brien after reading a series of critical tweets that the president of the International Brotherhood of Teamsters had sent about him in the past. “If you want to run your mouth, we can be two consenting adults. We can finish it here.”

The two men never came face to face in the hearing room. But they hurled insults at each other for around six minutes as Sanders repeatedly banged his gavel and tried to cut them off.

“You are a United States senator!” Sanders shouted at Mullin at one point.

Sanders, a longtime union ally, pleaded with Mullin and O’Brien to focus on the economic issues that were the subject of the Senate Health, Education, Labor and Pensions Committee hearing, which Sanders was holding to review how unions help working families.

Mullin, a frequent critic of union leadership, has sparred with O’Brien before. Earlier this year, O’Brien posted repeatedly about Mullin on X, formerly Twitter, calling him a “moron” and “full of s—” after Mullin criticized O’Brien at a hearing for what Mullin said were intimidation tactics.

In another social media post, which Mullin read aloud at Tuesday’s hearing, O’Brien appeared to challenge Mullin to a fight. “You know where to find me. Anyplace, Anytime cowboy,” O’Brien had posted.

The exchange escalated from there, with Mullin telling O’Brien that “this is the place” and asking if he wanted to do it right now.

“I’d love to do it right now,” O’Brien said.

Mullin replied: “Well, stand your butt up then.”

“You stand your butt up,” O’Brien shot back.

When Mullin got up from his chair, appearing ready for a fight, Sanders yelled at him to sit down, banged his gavel several times and told both of them to stop talking.

“This is a hearing, and God knows the American people have enough contempt for Congress — let’s not make it worse,” Sanders said.

As Mullin persisted, O’Brien retorted: “You challenged me to a cage match, acting like a 12-year-old schoolyard bully.”

The two traded angry insults for several more minutes — each called the other a “thug” — with Mullin at one point suggesting that they fight for charity at an event next spring, repeating an offer he made earlier this year on social media.

O’Brien declined, instead suggesting that they meet for coffee and work out their differences. Mullin accepted, but the two kept shouting at each other until the next senator, Maggie Hassan (D-N.H.), started her questioning by talking over them.

After the hearing, Sanders called the exchange “absurd.”

“We were there to be talking about, and did talk about, the crisis facing working families in this country, the growing gap between the very rich and everybody else, and the role that unions are playing in improving the standard of living of the American people,” Sanders said. “We’re not there to talk about cage fighting.”

Asked later about the skirmish, Senate Republican Leader Mitch McConnell of Kentucky demurred. “It’s very difficult to control the behavior of everybody who is in the building,” McConnell said. “I don’t view that as my responsibility.”

Sen. Kevin Cramer (R-N.D.) said that references were made to the back-and-forth in a GOP conference meeting after the hearing. But he said that no one should take it too seriously.

“It’s a dynamic place,” Cramer said of the Senate. “We don’t wear the white wigs anymore.”