Many legal experts believe that Trump's action was a violation of Federal campaign finance law and possibly Federal laws related to bribery and extortion. (Separate from that is consideration of whether, in this action, Trump launched both a conspiracy and a coverup.)
10. Not Impeachable: Finally, Republicans may ultimately argue that even if Trump's actions technically broke the law they are not of sufficient severity to constitute an impeachable offense. At the moment, that seems to be the attitude of most Republican Senators. (That's the position taken by former Attorney General Michael Mukasey on the October 3rd PBS News Hour ( .youtube.com/watch?v=e9fELgg6718 ).
Again, Democrats should return to the known facts of this case as detailed in the transcript of the Trump-Zelensky phone call and the whistleblower memo. (They can also use the recent statements of Donald Trump where he appears to be admitting to the accusations; taking the position that he is above the law.) Democrats have public opinion on their side and should press forward with impeachment proceedings.