The gloves were off as a fistfight nearly broke out between GOP Sen. Markwayne Mullin and Teamsters General President, Sean O'Brien, on Capital Hill Tuesday ... resulting in a stern scolding from Sen. Bernie Sanders.

Tensions erupted when ex-MMA fighter Mullin read out tweets by O'Brien following a previous heated congressional hearing, in which he was labeled a "clown and fraud," among various other insults.

With his damning gaze firmly on his opponent, Mullin warns of a "time and place to run his mouth" ... and as "two consenting adults," they could "finish it here."

The duo urges each other to "stand their butt up" to duke it out leaving an agitated Sanders to keep the peace and remind Mullin of his responsibilities as a United States senator.

"God knows the American people have enough contempt for Congress," Sanders says, clearly pissed about the showdown that was about to unfold.

Unrelenting, Mullin goes on to taunt his rival for "going quiet" ... only for O'Brien to hit back by likening him to a 12-year-old for challenging him to a cage match. Ouch.