Washington DC - Oklahoma Republican Markwayne Mullin challenged International Brotherhood of Teamsters General President Sean O'Brien to "stand your butt up" and fight during a Senate labor committee hearing on Tuesday.

O'Brien (51) appeared alongside fellow labor leaders Shawn Fain of the United Auto Workers and Sara Nelson of the Association of Flight Attendants-CWA, AFL–CIO during a Senate Health, Education, Labor and Pensions (HELP) Committee hearing entitled Standing Up Against Corporate Greed: How Unions are Improving the Lives of Working Families.

The three union presidents testified on the power of organized labor to uplift everyday Americans and curb the influence of powerful anti-worker corporate interests.

But there was at least one committee member who wasn't having any of it: Republican Senator Markwayne Mullin of Oklahoma.

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Mullin (46) apparently took offense to tweets posted by O'Brien after their last viral contentious exchange in March, in which the two traded punches over their respective salaries.

After reading out O'Brien's tweets, Mullin challenged: "You want to run your mouth? We can be two consenting adults. We can finish it here."

"Okay, that's fine, perfect. I'd love to do it right now," O'Brien responded.

"Then stand your butt up then," Mullin answered, to which O'Brien replied, "You stand your butt up."

The senator then got to his feet and appeared ready to advance toward the Teamsters president.

Senate HELP Committee Chair Bernie Sanders (82) immediately sought to de-escalate the situation, hitting his gavel and urging his Republican colleague to control himself.

"Sit down!" Sanders chastised Mullin. "You're a United States senator!"

"This is a hearing. And God knows the American people have enough contempt for Congress. Let’s not make it worse," he added.

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Mullin whined, "I don’t like thugs and bullies," as O'Brien quipped, "I don’t like you, because you just described yourself."

When Mullin got the floor, he repeated his demand: "Let’s do this because I did challenge you and I accepted your challenge. And you went quiet."

"I didn’t go quiet," O'Brien countered. "You challenged me to a cage match, acting like a 12-year-old schoolyard bully."

Sanders again sought to regain control of the hearing: "If you have questions on any economic issues, anything, go for it," he said. "We’re not here to talk about physical abuse."