Trump doesn't need to prove wrongdoing because the acceptance by Cokehead of $50,000-$83,000/month, and continuing to be paid, corresponding directly to the time that his father started running Ukraine (not a typo), is prima facie evidence of wrongdoing. That is the corruption standard that would start any grand jury's indictment in the US. That is the standard that HAS started multiple indictments in the past, like Bob Menendez in recent memory. If a politician or a member of the politician's family receives money with a temporal nexus to demands for a political favor it gets people thrown in jail. I had a client who is currently in jail at this moment based on an investigation started solely due to a temporal nexus (and a really thin suspicion at that). Hell, if that isn't the standard then the entire impeachment investigation was improper.

And this is the paradox: if the temporal connection is not sufficient then the call is proper because there is no evidence of wrongdoing, just indignation in the deep state that Trump wasn't listening to their "expert" opinion on Ukraine. However, if it is sufficient, then the request for the investigation of Biden was ALSO proper because no one has really conducted anything other than a pro forma investigation up to this point. By either standard Trump's call was proper.