"All the President's Defenses" 
https://www.lawfareblog.com/all-presidents-defenses

This article presents an analysis of two of the preliminary arguments that sources say President Trump's Lawyers have submitted to Special Counsel Robert Mueller providing challenges to potential obstruction of justice charges. The key constitutional argument presented is that, "because the President has the constitutional authority to hire and fire executive branch officials, Trump could not have obstructed justice when he fired FBI Director James Comey." As the article reports, critics find this defense wholly underwhelming. 
 Yet, there are still many important questions imbedded within this argument. 
Is it constitutionally allowable to act with an unconstitutional purpose? What types of conduct may not be criminally liable but are still not constitutionally permissible? Is this argument strategic at all, considering it rests on the principle that the president did act with nefarious intent? Lastly, was it the firing of Comey that constituted obstruction of justice or the fact that the former FBI director reported that Trump asked him to drop the Russia investigation?