My colleague Marty Redish has this oped in the New York Times, written before President Trump announced the pardon of Joe Arpaio. Noah Feldman had this oped on Bloomberg View. The pardon is now a fait accompli; even if it exceeded Trump's constitutional authority, nothing can be done about it, given that Arpaio's contempt citation cannot be enforced without the cooperation of the executive.
The remaining question is how much damage the pardon will do (if any) to Trump's reputation and ability to govern, or to the Republicans' electoral prospects in 2018 and beyond. In that regard, let me just point out that a much earlier generation of Americans took a very dim view of executive encroachment on the independence of the judiciary, as stated in the Declaration of Independence as the ninth "usurpation" of King George:
He has made Judges dependent on his Will alone for the tenure of their offices, and the amount and payment of their salaries.
As Redish and Feldman point out, Trump has undermined the very authority of the judiciary to enforce its orders. "Tenure of office" and "payment of salary" will not do much good if the executive feels free -- based on "his will alone" -- to reverse judicial rulings, especially in the sensitive area of civil rights enforcement.
It is hard to predict how much more broadly Trump will wield his pardon power to weaken the judiciary, but his latest usurpation was definitely a step in the wrong direction.