I wouldn't call it an abuse of power, necessarily.
If there was a mere POSSIBILITY of corruption on the Ukraine (which we can safely assume it was), I would say an US President, as an executive branch of goverment (espetially in cases like foreign policy), has an obligation to adress it with the Ukrainian president, in the name of the national interest.
It doesn't matter if the son of his political opponent might be somehow related to this case. It may be a bad optics, but we shouldn't treat polititians and their relatives like some holy cows. Remember, this whole affair with Trump, Guliani and Ukraine begins way before Joe Biden even announced, that he's running for office. I truly believe that Trump's main goals were to discover origins of 2016 interference on Ukraine in his campaign and maybe some Russiagate origins, among other things. Bidens are just a bonus in this.
To be clear, I don't have problems with Trump and Republicans under the loop of investigators. It's just bad that it looks like there are double standards in case of Democrats.