From the abstract for Mark Greenberg and Harry Litman, Trump’s Corrupt Use of the Pardon Power, Lawfare, June 19, 2018:
In no aspect of his presidency has Trump acted more immorally and done more damage to the rule of law and constitutional norms than in his exercise of the pardon power. He has granted only five pardons, far fewer than any modern president at a comparable stage. And the immediate practical consequences of these pardons have been modest relative to the havoc he has wreaked with other endeavors — for example, his all-out assault on federal law enforcement institutions. But his pardons violate core principles of justice and appear designed to settle scores with enemies and to encourage allies to resist cooperation with the Mueller investigation. At times, Trump appears almost gleeful about using the pardon power in a manner directly contrary to its purpose.
Three of his pardons — the recent surprise grant to conservative political commentator Dinesh D’Souza; the April grant to Scooter Libby, former adviser to Vice President Dick Cheney; and the pardon of Sheriff Joe Arpaio last August — involve grave abuses of presidential power. Each of these pardons was a debasement of the pardon power and an egregious affront to constitutional norms and the rule of law.