Don Lemon firmly believes President Trump is the reason Greg Gianforte body slammed a reporter -- and he flipped out on the air when someone dared to disagree with him.

Lemon had a panel on his show Wednesday night, and immediately stated HIS take on the Montana politician ... and clearly expected/wanted political commentator Paris Dennard to agree.

When Paris stood his ground ... Lemon went into such an attack mode, it's downright ironic.

At least this was just a verbal body slam.