I think the answer is that there needs to be proof of a conspiracy - a quid pro quo - for this to be impeachable. A crime against the state. Something like "you do this for me Vlad my lad, and this is what I'll do for you". Or what many think and is clearly asserted in a book I am finishing up called "Road to Unfreedom". Most of it has been a fascinating look at Russia's policy in Ukraine, and attending disinformation campaigns there and beyond designed to tear down the international institutions which keep Russia at a disadvantage in the world. Clearly very slanted against nationalism worldwide, and very much against Trump. The author, Timothy Snyder, considers it a fact that Russia has been behind Trump's branding business since it saved him from financial ruin in the early 1990s. In his view, as others here have espoused, Trump is a fiction (think "The Apprentice") largely created by Russia, and is their puppet. If that could be shown, that too would be impeachable IMO.

I don't buy any of that without proof. But here's the rub - Snyder considers it a fact that Trump won't release his tax returns because they would make clear the true source of Trump's wealth and confirm the Russian puppet theory. Assuming its not true, the best way to throw egg all over Snyder's face is to release the tax returns and prove him wrong.

It's not like that's unusual. Releasing tax returns has been the rule, not the exception, for half a century. Which is why I have no sympathy for Trump or his minions about the bad ol' media or the evil Mueller investigation. He brings all of it on himself, and if he has nothing to hide, he's the one who could prove it.