Round two between Teamsters President Sean O’Brien and Oklahoma Sen. Markwayne Mullin was sure a doozy.

The Senate floor on Tuesday almost transformed into a wrestling ring/UFC Octagon when the Republican senator challenged Charlestown’s O’Brien to an actual fight.

It never turned physical, but the shouting match led to Vermont Sen. Bernie Sanders getting in the middle of the battle as he started yelling and banging his gavel.

The beef between O’Brien and Mullin has been building for months. They went at each other during a March Senate hearing as they argued about unions, their salaries and union members’ dues. During the intense exchange, O’Brien said Mullin had “hid money” when he owned a company.

Then after that Senate hearing, O’Brien — a fourth generation Teamster who joined Teamsters Local 25 in Charlestown when he was 18 — tweeted several times about Mullin.

“After you left here, you got pretty excited about the keyboard,” Mullin told O’Brien during Tuesday’s hearing for the Senate Committee on Health, Education, Labor and Pensions.

He noted that O’Brien tweeted at him five times, and the senator showed the posts on printouts at the hearing.

“The last one (post) said, ‘Greedy CEO who pretends like he’s self-made,’ ” Mullin said.

“Sir, I wish you was in the truck with me when I was building my plumbing company myself and my wife was running the office, because I sure remember working pretty hard and long hours,” the senator said in response to O’Brien’s post.

Then Mullin read the rest of the post, ” ‘What a clown. Fraud. Always has been, always will be. Quit the tough guy act in these Senate hearings. You know where to find me. Any place, any time cowboy.’ ”

Then Mullin challenged him to a physical fight.

“Sir, this is a time. This is a place,” the senator said. “You want to run your mouth. We can be two consenting adults. We can finish it here.”

“OK, that’s fine,” O’Brien responded. “Perfect.”

“You want to do it now?” Mullin asked.

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

“Well, stand your butt up then,” Mullin told O’Brien.

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

The senator then stood up, and Sanders started yelling.

“Hold it! Stop it! Sit down!” Sanders said. “You know you’re a United States senator!”

The Democratic senator started banging his gavel, and told O’Brien that he couldn’t respond at that time.

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

Mullin kept trying to talk over Sanders.

“I don’t like thugs and bullies,” Mullin said of O’Brien.

The Teamsters president then responded, “I don’t like you because you just described yourself.”