President Trump and Obstruction

I applaud Prof. Alan Dershowitz's decrying the criminalization of political differences (NYT, Nov. 29,2017,P. A23) but strongly disagree with his conclusion that President Trump's attempt to stifle the Flynn investigation was, somehow, lawful. It is a federal crime to attempt to obstruct justice by corrupt persuasion which courts have defined as persuasion motivated by an inappropriate or improper purpose to convince another to engage in a course of behavior that impedes an ongoing investigation. By telling FBI Director Comey to discontinue his investigation of Michael Flynn, President Trump was attempting to impede that investigation for an improper purpose, namely to prevent exposure  of his own misdeeds or those of his campaign.

Yes, a president has vast executive powers but, unless one accepts Nixon's infamous pronouncement, that an act cannot be a crime if done by the president, Trump can be guilty of obstruction of justice and, thus, be subject to impeachment.