I rather suspect he won't release his tax records because he's the pot calling the kettle black. My gut says that if he didn't have anything to hide, he wouldn't need that bit of extraordinary privacy (extraordinary for a presidential candidate, anyway).
This is one of those rare exceptions where that phrase seems pretty accurate, particularly given how he has bragged about having politicians in his pocket. And, if convicted of bribery, that's grounds for automatic impeachment. Just saying.