Mark Osler is the Robert and Marion Short Professor of Law at the University of St. Thomas in Minnesota, and the author of "Contemporary Criminal Law" (West, 2018). The opinions expressed in this commentary are his own.

(CNN) With his sixth use of clemency, President Trump is pardoning Dinesh D'Souza, a conservative author and filmmaker. In 2014, D'Souza pleaded guilty to violating campaign finance laws and received a sentence of five years of probation, including eight months of supervised living in a halfway house. With this grant, this President's use of clemency is becoming clear. Trump seems to be employing the pardon power as much for retribution as for mercy — to lash out at his critics and reward his friends who are under attack.

The constitutional pardon power is a rare and remarkable thing: It gives the president nearly unchecked power to relieve the burdens of a criminal conviction. It was meant to be a tool of mercy; Alexander Hamilton described it as such in Federalist 74.

It can play a unique and important role in sanding off the roughest edges of criminal justice. It has been abused by other presidents, of course; as a former federal prosecutor I still bristle when I think of the pardon Bill Clinton gave to Marc Rich , a fugitive who had been convicted of multiple counts of tax evasion.

Yet, while it is nothing new for a president to use clemency to reward friends and family, there is something unique and darker taking shape. Trump seems to be using the pardon power not only for the sodden purpose of helping buddies, but also to hurt those who have opposed him.

Take D'Souza, for example. In his tweet announcing the pardon, Trump said "Will be giving a Full Pardon to Dinesh D'Souza today. He was treated very unfairly by our government!" While it might be unclear how a sentence of probation is "very unfair" to someone who pleaded guilty to a felony, it is clear who was dishing out that supposed unfairness: Preet Bharara, the former United States attorney who prosecuted D'Souza for his crime. Trump, of course, has had a running feud with Bharara : Trump fired Bharara as the United States attorney for the Southern District of New York, and Bharara then emerged as a consistent critic of Trump.

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