Tom Arnold is DESTROYING Donald Trump for his war on the NFL ... and tells TMZ Sports he knows the real reason POTUS is going after the league and its players.
Tom says Trump's still pissed about a failed attempt to buy the Dallas Cowboys back in the day ... a business move that could've meant billions.
"He's mad because they wouldn't give him a franchise.”
"They wouldn’t let him do it and he holds resentment ... and it’s ridiculous.”
Arnold wasn’t done there – telling us Trump’s a “stupid idiot” for pitting his cult of "morons" against players protesting during the national anthem.