Trump’s staff heavily edited the transcript of his telephone call with Ukraine’s president in which he illegally extorted assistance with his reelection campaign. More than half of the transcript is missing:
Text messages show that Trump and his staff pressured Ukraine’s government to assist Trump’s reelection campaign by investigating his political opponent:
https://www.cnn.com/2019/10/03/politics/us-envoys-trump-ukraine-investigate/index.html
There were many other questionable telephone calls that need to be examined:
https://www.theguardian.com/us-news/2019/oct/02/trump-transcripts-impeachment-inquiry-ukraine-nixon
Update: Trump’s chief of staff has admitted that Trump withheld nearly $400 million in U.S. military assistance to Ukraine unless it investigated his political opponent:
https://www.nytimes.com/2019/10/17/us/politics/trump-mulvaney-ukraine.html