https://www.washingtonpost.com/opinions/trump-has-done-plenty-to-warrant-impeachment-but-the-ukraine-allegations-are-over-the-top/2019/09/20/51eff90c-dbf1-11e9-bfb1-849887369476_story.html
Trump held up the delivery of $250 million in military assistance to Ukraine, which is under constant threat from neighboring Russia. He had a phone conversation on July 25 with President Volodymyr Zelensky of Ukraine. According to the Ukrainian government, the call included a discussion of Ukraine’s need to “complete investigation of corruption cases, which inhibited the interaction between Ukraine and the USA.”
So it appears that the president might have used his official powers — in particular, perhaps the threat of withholding a quarter-billion dollars in military aid — to leverage a foreign government into helping him defeat a potential political opponent in the United States.
If Trump did that, it would be the ultimate impeachable act.