David Cay Johnston, investigative reporter and author of The Making of Donald Trump, said:

Why is it that Donald Trump is so insistent that we’re not to see his tax returns? There must be something, a hiding, in his returns. I’ve been at this for 50 years. I started as a teenager doing investigative reporting. And every time some high-level politician wants to hide something, it always turns out there’s a reason, they’ve got something to hide. Lots of things we can think of that Donald Trump has to hide, including sources of income and his connections to the Russian oligarchs, who are essentially a state-sponsored network of international criminals. And you’ve got to think about them that way and understand that’s their position in the world. And Donald Trump really is desperate that we don’t see where his money comes from.