I think one big problem is that the Trump Tower Moscow deal may have involved financing by a Putin owned Russian bank that was under US sanctions.

At the very least, dealing with such a bank for financing of Trump, could have rendered "blackmail" worthy communication streams. 



Considering Trump's apparent cover up, denials, and lack of disclosure of the potential deal (which he himself apparently signed off on - signing a letter of intent negotiated by his most prominent lawyer/talking head/campaign surrogate).

In the worst case, American law may have been violated, or, US policy negatively affected (in ongoing fashion) in favor or Russia's national interests, by blackmail over the record of prior dealings.   

That takes a history of ongoing Trump Tower Russia dealings outside of the realm of "child's play".

Has become a never ending source of amazement to me to watch Trumpist sheep try to create justifications and rationales for Trump's misconduct.