With the September 24th initiation of a formal impeachment inquiry, the political battle lines have formed. Democrats will subpoena witnesses and gather material that will be presented before the House Intelligence committee; eventually the House Judiciary Committee will construct the formal impeachment measure and submit it to the entire House of Representatives. Meanwhile, Donald Trump, and his Republican acolytes, will do everything they can to discredit the inquiry. Their obstruction will take (at least) ten forms.
By the way, before you consider what follows, it would be a good idea to read the "Unclassified Memorandum of Telephone Conversation" between Donald Trump and Ukrainian President Zelensky (https://www.politico.com/story/2019/09/25/trump-ukraine-phone-call-transcript-text-pdf-1510770 ). It would also be a good idea to read the "Unclassified Whistleblower memo to Senator Richard Burr and Congressman Adam Schiff." (https://www.nytimes.com/interactive/2019/09/26/us/politics/whistle-blower-complaint.html)
One Trump strategy will be to ignore the allegations of Trump misconduct and to attack.
1.Harassment: Trump loyalists will insist that Democrats have been "hounding" Trump for three years and this impeachment inquiry is the latest example of unfair treatment. Republicans will assert, "Democrats aren't interested in governing; they spend all their time attacking Trump." Republicans won't address any of the specific accusations against Trump but rather demean them as "more of the same" and claim that Dems are trying to "steal the election." (That's the theme of the latest Trump campaign ad (https://www.realclearpolitics.com/video/2019/09/27/trump_campaign_ad_democrats_want_to_impeach_trump_for_trying_to_drain_the_swamp.html).)
2. Setup: Some Trump supporters will go into more detail and assert that the Trump-Zelensky affair was "a set up." These Trump stalwarts will, in essence, be claiming that Democrats fabricated the phone conversation and the related whistleblower information. (That's the drift of the claims by Republican stalwart Liz Cheney (https://www.usatoday.com/story/news/politics/2019/09/30/liz-cheney-ukraine-phone-call-political-set-up-donald-trump/3826791002/).)
3. Vendetta: Other Trump supporters will claim that the impeachment inquiry is a personal vendetta being lead by House Intelligence Committee chair Adam Schiff. Republicans will personally attack Schiff -- Trump has already called Schiff a traitor and urged him to resign. (On September 29th, Trump tweeted, "I want Schiff questioned at the highest level for Fraud & Treason.")
4. Deep State: Many Trump supporters will assert that the inquiry is a conspiracy launched by "the deep state" -- that is, by the intelligence community including the CIA and FBI. Many Trump loyalists have long claimed that elements within the intelligence community have been out to get Trump since he entered the White House. (Recently, Republican stalwart Newt Gingrich wrote, "[The impeachment inquiry] is a legislative coup d'etat. It is an effort by the hard left, the news media, and the deep state to destroy the president chosen by the American people," )
5. Biden: Finally, some Trump advocates will take the position that not only is there nothing to the Trump-Zelensky affair but rather the mainstream media is missing the real story: Joe and Hunter Biden's illegal involvement in Ukraine. (On September 29th, White House staff member Stephen Miller claimed "Trump is the real whistleblower." (https://www.foxnews.com/politics/stephen-miller-says-trump-is-real-whistleblower).)
For each form of these attacks, the Democrats response is straightforward: they should return to the known facts of this case as detailed in the transcript of the Trump-Zelensky phone call and the whistleblower memo. The problem Dems face is that Trump is launching a multi-million dollar attack campaign over social media (https://www.cnn.com/2019/09/30/politics/facebook-trump-impeachment/index.html). Democrats have to make sure that their side of the story is widely publicized.
Another core Trump strategy will be to attempt to undermine the whistleblower report.
6. Hearsay: Republicans will assert that the whistleblower report is based on "hearsay;" that is, it is inaccurate, because the whistleblower was not present during the actual phone call(s).
The Democratic response should be to point out that the Inspector-General has already conducted an investigation and has corroborated the whistleblower assertions.
7. Illegal Act: Another way to demean the whistleblower claim is to assert that he or she broke the law. That is, regardless of the facts of the matter, Republicans will claim that the information was obtained illegally. (On September 30th, Trump tweeted: "The Whistleblower knew almost nothing, its 2ND HAND description of the call is a fraud!")
Again, the Democratic response should be to lean on the report of the Inspector-General, who has determined that the whistleblower acted within the law.
8. Obstruction: The Trump Administration can seek to undermine the whistleblower report by blocking Congressional verification of the elements of the report; that is, keeping congressional committees from recreating the investigation already conducted by the inspector general. That seems to be the strategy of Secretary of State Mike Pompeo (https://talkingpointsmemo.com/muckraker/pompeo-house-depositions).
9. No Crime: A more sophisticated tactic would be for Republicans to argue that no crime was committed. That is, Donald Trump may have asked Ukrainian President Zelensky to for a "favor" -- to investigate the Bidens -- in return for military aid but that is not a violation of the law. On October 3rd, Trump seemed to take this position when he asked both Ukraine and China to continue to investigate the Bidens. (https://www.cbsnews.com/live-news/trump-impeachment-inquiry-latest-updates-today-2019-10-03/)