Quid pro quo, means this for that. I'll send you the military aid, once you announce an investigation, a "favor." Trump asked for it, because he directly benefits from it. It helps his reelection campaign if his opponent is under investigation. Hopefully we can all see that. Quid pro quo isn't illegal in and of itself, the US does it all the time. But what Trump was asking interferes in the US's political process. SO forget about quid pro quo, Trump solicited help from a foreign nation. He explicitly did it, requesting that Zelensky look into the Bidens and the debunked conspiracy theory that Ukraine interfered in the 2016 election. It is accepted by the entire intelligence community that Russia was behind it. SO regardless of quid pro quo, it is illegal for a candidate or campaign to "solicit" help from a foreign gov't, whether it's money or something else of value. Bribery is also illegal, and grounds for impeachment. "specific intent to give or receive something of value (Biden investigation) in exchange for an official act (meeting at WH, military aide).
How is an investigation a thing of value when you don't know the outcome? Imagine this. Trump asks for an investigation, it shows Biden was above board in all his actions, that actually backfires on Trump. I am of the opinion that an investigation where the outcome is unknown has no political value one way or another So that's just one legal hurdle.
Here's another. If it is accepted that an investigation, the results of which are not known, is a thing of value, you have to establish beyond a reasonable doubt that Trump did it for political reasons.
That is where the pesky treaty comes into play. He can always point to it and say, this is why I did it. I was duty bound to do so.
