re: Specifically, how did President Trump abuse his power concerning Ukraine?

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The abuse thing happened when he tried to override Congress and withhold military aid.

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No two ways about it, that's just flat out wrong. Withheld aid with the unambiguous intention of receiving personal gain via pressuring Ukraine to investigate a political rival is intrinsic to the abuse of power/withholding aid impeachment count.

The temporary suspension of aid, standing alone, is a non-starter and even Pelosi et al don't dispute that.

So then you're back to square one, proving Trump's state of mind and inner intentions. And please feel free to provide any evidence of that, i.e. unambiguous evidence such as a direct statement by Trump to the effect of "You know what? Despite the official position, I was really intending to get dirt on Biden to help with my reelection campaign." Direct statement, not the feeling that other people may have had. Because that's lay witness opinion and holds no weight if you're being calm and objective about it all.