No matter how polite, informal, and non-demanding the ask is - and no matter who the foreign President it - it's still POTUS leveraging the unique power of that office to focus a foreign government's investigative arm at specific US citizens.
I'm not saying that in all cases that is an abuse of power. Every situation should be evaluated on a case-by-case basis. However, in a case like this, I don't think the burden-of-proof falls on the accuser. When using official powers, and singling out individuals who just happen to be your political rivals, I think the burden now falls on the official to provide a cogent explanation and defense of their actions. This isn't a criminal prosecution, it's an examination of whether or not POTUS is using his office towards personal ends.