Sen. Markwayne Mullin has challenged a labor union leader to a charity MMA fight after their head-to-head matchup on Capitol Hill spilled over onto social media.

Senator Mullin made the challenge after Sean O’Brien said on Twitter, “You know where to find me. Anyplace, anytime cowboy.”

O’Brien also called Mullin “a clown & fraud.”

The Twitter exchange comes months after Senator Mullin and O’Brien argued during a hearing in Washington. Senator Mullin has suggested that the fight take place in Tulsa on September 30.

The funds will go towards, “a great cause,” but that cause was not shared.

During a Senate committee meeting in early March, Sean O’Brien, the general president of the International Brotherhood of Teamsters, called Mullin a “greedy CEO” amid a heated discussion between the two.

Then, on June 21, O’Brien tweeted a video of Mullin mentioning the labor union leader during a more recent committee hearing. In that tweet and a series of others, O’Brien called Mullin a “moron” and a “clown & fraud” and seemingly challenged the U.S. senator from Oklahoma to a fight.

O’Brien continued to call out Mullin on Twitter.

“What have you done for working people in OK @SenMullin? Last time I checked, your state ranks near the bottom in median wages,” O’Brien tweeted. “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. #Truth”

O’Brien has not responded to Mullin’s challenge.