GOP Sen. Markwayne Mullin challenged Teamsters president Sean O'Brien to a physical fight during a Senate hearing on labor unions.

The confrontation stemmed from a heated exchange in March over Mullin's business background and O'Brien's union salary.

Mullin referred to O'Brien's tweets calling him a "greedy CEO," a "clown," and a "fraud."

Holding printed versions of O'Brien's tweets, Mullin invited him to settle the dispute on the Senate floor.

O'Brien agreed, prompting Mullin to stand up, but Chairman Bernie Sanders intervened, reminding Mullin of his role as a U.S. senator.

Mullin, a former undefeated MMA fighter, and O'Brien continued verbal sparring, agreeing to meet for coffee at one point.

The situation escalated with insults, leading to Sanders banging the gavel to restore order.

Mullin later told reporters the confrontation wasn't personal, accepting O'Brien's challenge without harboring hard feelings.