Less and less does it sound like concern for his ability to perform, and more just straight out persecution. I never cared if he released his taxes, since many other politicians refuse to do so -- Trump is not the _ONLY_ one, and many of them are Democrats. Truth be told, tax returns for businesses don't show much... Don't know if you've ever seen one, but it won't tell you anything about what they were doing for the most part -- it's mostly "profit and loss" sort of thing. It usually will not tell you the exact nature of expenses, loses, etc, just that they happened. That's exactly why I'll keep telling you there is nothing there... I've filled out those forms for years and they barely contain anything relevant. But, that being said -- if he wants to keep them private I certainly think it's his right.
How does Trump commit financial crimes, btw? He personally does nearly nothing in that regard himself -- he has a banker, accountant, lawyers, and so on that handle all his money and have power of attorney over those transactions. (In most cases...) If anyone went to jail for a crime it'd be them, not him. He'd have only followed their advice; good or bad.