Sen. Markwayne Mullin (R-Okla.) challenged the president of the International Brotherhood of Teamsters on Monday to an MMA fight for charity, reigniting the animosity between them that was on full display during a viral moment at a Senate hearing in March.

“An attention-seeking union Teamster boss is trying to be punchy after our Senate hearing. Okay, I accept your challenge. MMA fight for charity of our choice. Sept 30th in Tulsa, Oklahoma. I’ll give you 3 days to accept,” Mullin wrote in a tweet Monday. 

Mullin, a former mixed martial arts fighter, responded to a tweet from union president Sean O’Brien, who called Mullin a “clown” and a “fraud” and challenged him to “quit the tough guy act” and find him “any place, anytime.”

“Greedy CEO who pretends like he’s self made. In reality, just a clown & fraud. Always has been, always will be. Quit the tough guy act in these senate hearings. You know where to find me. Anyplace, Anytime cowboy,” O’Brien tweeted several days ago. 

In O’Brien’s tweet, he appeared to be mocking Mullin for his height, attaching a photo of Mullin using a step behind a podium and circling the step beneath Mullin’s feet in the photo. 

At a Senate Health, Education, Labor and Pensions Committee hearing in March, O’Brien told Mullin — who owned nonunion plumbing companies before selling his majority shares in 2021 — that Mullin was “out of line” for accusing him and other union leaders of “sucking the paycheck” out of union workers to pay for what Mullin described as the union leaders’ “exorbitant” salaries.

Mullin pushed back saying, “Don’t tell me I’m out of line,” and, “You need to shut your mouth.” 

The Hill reported O’Brien’s salary was roughly $193,000 in 2019; Mullin, meanwhile had a net worth $31.6 million and $75.6 million in 2020, according to the newspaper Tulsa World.

The latest development, however, seems to come from a moment in a more recent hearing, when Mullin was making a similar argument about union bosses taking money from workers. He mentioned O’Brien as an example, though Mullin appeared to forget O’Brien’s name, calling him O’Malley instead.

“Hey, JohnWayne Mullin..First off, my name’s O’Brien not O’Malley. Secondly, you should get your facts straight because every time you speak in these hearings you’re full of sh*t. The more you run your mouth, the more you show the American public what a moron you are,” O’Brien responded in a tweet with a clip of the hearing. 

He followed up with two more tweets directed at Mullin: one to which Mullin responded by challenging O’Brien to a fight and another tweet that said, “What have you done for working people in OK @SenMullin? Last time I checked, your state ranks near the bottom in median wages. Sounds like you need to shut your mouth & get to work for the people of your state. They deserve action, not your phony “man of the people” spiel.”