A tense moment ensued in Congress after Sen. Markwayne Mullin (R-OK) challenged Teamsters President Sean O'Brien to a fight during a meeting of the Health, Education, Labor and Pensions Committee.

"This is a time, this is a place to run your mouth. We can be two consenting adults, we can finish it here," Mullin said,  reading a June 21 tweet from O'Brien in which he called the Republican Senator "a clown & a fraud."

"Quit the tough guy act in these senate hearings," the tweet continues. "You know where to find me. Anyplace, Anytime cowboy."

"We can finish it here," said Mullin, a former mixed martial arts fighter.

"OK that's fine, perfect," O'Brien replied.

"You wanna do it now?" Mullin shot back.

"I'd love to do it right now," O'Brien replied.

"Well stand your butt up then," Mullin replied - to which O'Brien echoed, "You stand your butt up!"