A White House memo recording U.S. President Donald Trump's July phone conversation with Volodymyr Zelenskiy is damning.
Trump's request that the president of Ukraine initiate a corruption investigation into Joe Biden and his son Hunter wasn't incidental. On the contrary, it appears to have been the point of the call, along with an additional request to investigate the origins of the Russian collusion allegations against Trump. Trump brought up the investigations nearly every time he opened his mouth. Zelenskiy responded positively, suggesting he got the point.
There is more than enough evidence here to support an allegation that Trump was not merely asking the president of Ukraine "to do us a favor," as he put it, but rather proposing a quid pro quo in which U.S. aid for Ukraine would be reinstated in exchange for an investigation into the Mueller investigation, and into Biden. That would constitute an abuse of power by the president of the United States for his own benefit, since Biden was and is the leading contender for the Democratic nomination to challenge Trump in 2020.