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The Donald is still going to bat for his boy Tom Brady -- opening up a rally in Rhode Island Monday by demanding, "LEAVE TOM BRADY ALONE!"

Trump has been a Brady supporter for a while -- and backed his longtime friend after the initial "Deflategate" suspension last year.

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So now, with an appeals court ruling that Brady must serve the penalty -- Trump began his rally this morning with another staunch defense of the Patriots QB.

The brofest isn't one-sided ... remember, Brady had Trump's campaign hat proudly displayed in his locker ... though he has not given Trump his official endorsement.