A face-off has taken place during a US congressional hearing between an Oklahoma Senator and a union boss.
The pair each dared one another to "stand your butt up", leading to a heated quarrel the chair of the Senate labour committee, veteran Democrat Bernie Sanders, struggled to contain.
"Sit down!" Sanders yelled at Oklahoma Senator Markwayne Mullin, seated beside him. "You're a United States Senator!"
It came after Mullin, 46, called president of the Teamsters union Sean O'Brien a "thug".
O'Brien, 51, said Mullin was acting like "a schoolyard bully".
The tension kicked off during the Tuesday hearing after Mullin chose to read aloud a tweet O'Brien sent earlier this year, after another committee-room confrontation.
In the tweet, O'Brien had described Mullin as "greedy", a "clown" and a "fraud".
"Quit the tough guy act in these Senate hearings. You know where to find me," the tweet read.
"You want to run your mouth? We can be two consenting adults, we can finish it here," Mullin said, looking across at O'Brien.
"OK, that's fine - perfect," O'Brien responded. "I'd love to do it right now."
"Then stand your butt up then," Mullin said. "You stand your butt up," O'Brien replied.
Mullin then stood up, before Sanders directed him to sit back down.
Despite Mullin sitting down, the two men continued to quarrel - causing Sanders to bang his gavel.
"This is a hearing and God knows the American people have enough contempt for Congress, let's not make it worse," Sanders said.
"If you have questions on any economic issues, anything, go for it.
"We're not here to talk about physical abuse," he said.
Mullin went on to say he wanted "to expose this thug for who he is".
"Do not point at me, that is disrespectful," O'Brien said.
"I don't care about respecting you at all," Mullin snickered.
Shouting, "Hold it, no!" Sanders banged his hammer for the final time.
