Mitt Romney Heckles Russell Westbrook at Jazz Game

Mitt Romney ... talking crap?!

Yep -- it all went down at the Jazz game Monday night where a man running for senator in Utah sat on the court to watch his team take on the OKC Thunder in Game 4 of their playoff series.

When Russell Westbrook racked up his 4th foul in the first half ... Romney (wearing his own customized Jazz jersey) started talking smack ... heckling Russ from a really close distance!

Good news for Mitt ... the Jazz were able to close it out and eventually won the game 113-96 ... taking a 3-1 lead in the series.

Game 5 is Wednesday in Oklahoma City ... wonder if Mitt will fuel up the jet and keep the rivalry going?