A Republican senator who nearly came to blows with a national union boss told FOX News the Teamsters leader he nearly sparred with in a hearing is a "thug" who is all talk and no action.

Sen. Markwayne Mullin of Oklahoma has traded verbal barbs in the past with Teamsters President Sean O'Brien, but the situation escalated Tuesday after Mullin read incendiary tweets from O'Brien – including one in which the union boss intimated he could fight the lawmaker.

Mullin read a tweet from earlier this year in which O'Brien called him a "greedy CEO" with a "tough guy act." The lawmaker previously started and grew a plumbing business in Oklahoma but has been critical of how unions operate and argues they put their leaders' salaries above the wellness of workers.

Mullin told O'Brien he wished he was in his plumbing truck when he and his wife ran a two-person plumbing operation at first.

Mullin OBrien

Sen. Mullin and Teamsters President O'Brien (Fox News)

"Any place, any time, cowboy," he read from O'Brien's tweet. 

"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," he said in the hearing.

Mullin told "Hannity" that as he rose from his chair and began to take off his wedding ring, O'Brien looked terrified.

"I'm a guy from Oklahoma first, and in Oklahoma you don't do this. Maybe you run your mouth in New Jersey," he said. "But this is some thug that's a mob boss. And you're supposed to be intimidating because he's the boss of the Teamsters-- and he's gotten away with this.

"You're not going to run your mouth at me and expect me to sit there and -- you should have seen the fear in his eyes when I stood up, too. And I'm not joking," Mullin, who is also a former MMA fighter, continued.

Mullin said he was not looking for a fight, but that O'Brien has a history of intimidating behavior, citing a 2013 report about him facing suspension while president of the Teamsters Local of Charlestown, Mass., in connection with alleged intimidation of members in a Rhode Island union election.

In another interview with Oklahoma's ABC affiliate, Mullin added Democratic President Andrew Jackson participated in nine duels, including one where he "knocked a guy out in a dinner at the White House."

Earlier on FOX News, host Jesse Watters additionally cited an 1856 incident in which pro-slavery Democratic Congressman Preston Brooks of South Carolina nearly killed abolitionist Massachusetts Republican Sen. Charles Sumner by caning him to death in the Capitol.

On "Hannity," Mullin later extrapolated that union bosses like O'Brien are often on the dais because Sanders, as Senate HELP Committee chairman "is a socialist who runs our committee."

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"He's in the unions' pockets, or the unions are in his pocket, one of the two," he said.

"I'm not against unions. Fine. If you want to join a union, join a union. But there's a reason why, 50 years ago, 33% of the labor force was in a union and today, it's 10%. And there's a reason why Right To Work states like Oklahoma have a $4,000 a year salary higher than union states."

Host Sean Hannity said unions tend to take members' monies and distribute it among left-wing political interests.

"[C]hoice matters and people want to make a decision on how they work and where their money goes," Mullin added.

Following the confrontation, O'Brien said Mullin is an "embarrassment to the state of Oklahoma."

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