WASHINGTON (KFOR) — U.S. Sen. Markwayne Mullin, R-OK, has challenged the general president of the International Brotherhood of Teamsters to a mixed martial arts fight on Twitter.

Mullin sits on the Senate Committee on Health, Education, Labor and Pensions.

In March, Teamsters General President Sean O’Brien testified in front of the committee.

During that hearing, Mullin and O’Brien had a heated exchange on unions and intimidation, during which Mullin said, “I’m not afraid of a physical confrontation, in fact, sometimes I look forward to it.”

On June 21, O’Brien tweeted about Mullin several times, including a tweet that reads, “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. #LittleManSyndrome”

Mullin replied June 26 with a challenge to an MMA fight.

In a tweet Monday morning, Mullin said, “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.”

O’Brien was not tagged in the tweet, and it is unclear whether he is aware of Mullin’s challenge.