Mr Mullin, a former mixed martial arts fighter, got into a heated argument with the President of the International Brotherhood of Teamsters Sean O’Brien on Tuesday that was close to becoming physical.
Mr Sanders, the chairman of the Senate Committee on Health, Education, Labor and Pensions, and a two-time Democratic presidential nominee runner-up, stepped in and admonished Mr Mullin.
The 82-year-old Mr Sanders, an independent US Senator from Vermont, was leading a hearing centred around how unions can help working families.
Mr Mullin read a tweet posted by Mr O’Brien in June in which the union boss called the senator a “clown” and a “fraud” before going on to say that he “pretends like he’s self-made”.
“In reality, just a clown & fraud. Always has been, always will be. Quit the tough guy act in these senate hearings. You know where to find me. Anyplace, Anytime cowboy,” he wrote at the time.
“Sir, this is a time, this is a place. You want to run your mouth, we can be two consenting adults, we can finish it here,” Mr Mullin told Mr O’Brien during the hearing.
“Okay, that’s fine. Perfect,” the union leader responded.
“You want to do it now? Stand your butt up then,” the Oklahoma senator said, standing up.
“Hold it. No, no, no, sit down. Sit down! You’re a United States senator, sit down,” Mr Sanders said.
As the bickering carried on, the chairman added: “Excuse me, hold it. Senator Mullin, I have the mic. If you have questions on any economic issues, anything that was said, go for it. We’re not here to talk about physical abuse.”
Mr Mullin faced online mockery after almost coming to blows with the union boss.
“Women are too emotional to hold public office,” podcast host Allison Gill wrote on X.
Ben Collins of NBC News simply wrote: “Normal stuff, normal country!”
The widely followed “Angry Staffer” account added: “Senator Mullin hid under a chair on January 6, but he’s real tough when he knows nobody is going to let him fight a witness.”
The Rightwing Watch watchdog noted that “Last month, Sen. Markwayne Mullin told a group of religious-right activists that as a Christian, he is called to love his political opponents: ‘Even if you disagree with me, I still love you, I still respect you.’”
Last month, Sen. Markwayne Mullin told a group of religious-right activists that as a Christian, he is called to love his political opponents: "Even if you disagree with me, I still love you, I still respect you." https://t.co/I4UheT4Nqf pic.twitter.com/YmFdI5fwEm
— Right Wing Watch (@RightWingWatch) November 14, 2023
Former Hillary Clinton and Pete Buttigieg strategist Jennifer Holdsworth tweeted out “a reminder that big time coward @SenMullin hid in fear from the insurrectionists he helped foment. He’s a disgrace to the Senate and to the country”.
— Jennifer Holdsworth (@HoldsworthJen) November 14, 2023
“Kevin McCarthy pushes into a colleague who voted against keeping him as speaker. Now this from a GOP Senator. Don’t tell me Trump isn’t normalizing violence in our civil discourse. This is how it starts,” Joyce Alene said.
Mr O’Brien appeared on CNN on Tuesday afternoon, where he was asked if he was concerned that his tussle may have drawn attention away from the real issues being discussed.
“Not at all. I mean, we had some great dialogue about issues that affect working people about organizing collective bargaining – unionizing America,” he responded. “And there was a lot of different opinions, a lot of different debate. And it was great until Markwayne Mullin – obviously we're renting space in his head – decided to erupt.”
“You know, we do have a lot of differences of opinions on both sides. But the one thing that's clear is that today's dialogue outside of what happened was fruitful, productive, and the one thing we've been doing as a teamsters union, I do represent 1.3 million members and I do it in the best interest of my members ... reaching across the aisle to find common ground on issues that can help working people throughout America. And we're going to continue to do that.”
“Unfortunately today, Mark chose to not act like a US senator and he's gonna have to pay the consequences for that,” Mr O’Brien said.
The Independent has reached out to the office of Mr Mullin for comment.