I think the problem for President Trump is that presidential candidates for decades have submitted their tax returns for several years. When I ran for political office I had to list various things, including real estate I owned, which I would have preferred to not declare to the world for fear of vandalism, etc.
Candidates have voluntarily submitted returns, but there is no requirement the returns are disclosed.
IIRC, the only disclosure is a financial statement which shows assets within a range. If there was an underlying criminal probe, that would be a different matter, but just disclosure so Dems can look for political advantage is exactly the same conduct they are trying to impeach him for.