Arnold is clearly reacting to Trump's blustery tweet, in which he says Schwarzenegger got creamed in 'Celebrity Apprentice' ratings when you compare them to what the Prez-elect got in his debut.

Schwarzenegger just responded, by tweeting, "I wish you the best of luck and I hope you'll work for ALL of the American people as aggressively as you worked for your ratings."

As we reported, the reality is Trump got huge ratings for his debut, but by his last season his rating for 'Celeb Apprentice' was roughly the same as Arnold's debut.

Arnold followed up with, "There's nothing more important than the people's work." Translation -- you shouldn't be involved in a ratings war.