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President Barack Obama used the pardon power to free hundreds of narcotics prisoners serving harsh terms. It was a principled use of mercy, a striking vision of the most powerful person on Earth recognizing the humanity of the least powerful. Now, President Donald Trump appears to be considering an abuse of that same power by helping some of the most powerful people on Earth, including himself. If he does try to stymie the investigation into collusion with Russia through clemency, Trump will harm both his legacy and the moral force of the pardon power itself.

The Constitution's pardon power clause imported a tool of kings into a democratic system, to allow for some measure of mercy in cases where "justice would wear a countenance too sanguinary and cruel," as Alexander Hamilton put it in Federalist 74. While no express checks and balances apply to clemency, the powerful forces of politics and shame historically have served as the often-effective limitations on its abuse. Presidents tempted to help undeserving friends or themselves have often (though not always) been deterred by the fear that they would not win another election or that they would disgrace their legacy.

Unfortunately, neither shame nor politics seems to apply to Trump as they do to others, and he may soon abuse the pardon power in historic ways. No one, even his supporters, would argue that Trump is dissuaded from words or action by a sense of shame. As for politics, this is a man who before the election bragged that he "could stand in the middle of 5th Avenue and shoot somebody" without losing his base voters. Subsequent outrages and the reaction from his supporters have largely borne this out as true. Meanwhile, a partisan Congress in the hands of Republicans seems unlikely to impeach and convict Trump.

Could Trump pardon himself pre-emptively? Of course he could; all he needs to do is execute a pardon warrant. No one else needs to sign off on it, and no congressional or court action is needed. He is free to ignore the protests of his pardon attorney and others in the usual chain of decision. The idea of pre-emptive pardon has precedent, too, most clearly when President Richard Nixon was pardoned by President Gerald Ford before any charges were laid. Such a pardon could be challenged in the courts, but that presents difficult questions of standing regarding who has the ability to sue. Moreover, if standing were established, it would end up before a Supreme Court that resolved the last such crisis in favor of President George W. Bush over Vice President Al Gore.

If Trump were to pardon himself, that would not end the investigation of Special Counsel Robert Mueller. Even after a pardon, Mueller could produce a damning report and seek charges from a grand jury against others in the administration. An alternative strategy would be to selectively pardon those who are cooperating or may cooperate with the investigators.

Federal criminal law is built around encouraging co-conspirators to testify against their co-defendants. Those who provide "substantial assistance" are eligible for a break under the federal sentencing guidelines, and mandatory minimums in drug cases do not apply to them. Prosecutors are trained to take a "proffer" from cooperators, assessing their worth and reliability, before moving on to use them for crucial information and key testimony. Conspiracies happen in secret, inside a box, and cooperators allow prosecutors eyes inside of that box. Were Trump to strategically pardon those who are cooperating or who may cooperate, such as Michael Flynn, he would be removing the threat of a long sentence that would motivate them to cooperate. With no hammer over their head, there would be no deal with Mueller.

Were Trump to pursue either of these avenues, or both, it would be a deeply tragic day. At its best, the pardon power was used by George Washington to pardon the leaders of the Whiskey Rebellion, by Abraham Lincoln to show mercy to Army deserters, by Harry Truman to spare the life of a man who sought to kill him, and by Ford to pardon not only Nixon but more than 13,000 draft evaders and deserters from the military during the Vietnam conflict. In its ability to heal the nation and offer mercy to the hated, clemency is the soul of the Constitution.

More recently, of course, presidents have misused and neglected the pardon power. President Bill Clinton and both Bushes largely ignored the constitutional imperative to show mercy, and when they did it was too often used to help cronies such as Marc Rich and Scooter Libby. That changed with Obama, though. In the last few years of his presidency, more than 1,700 men and women were released from overly harsh narcotics sentences with a stroke of the presidential pen in a project that was imperfect but true to the pardon power's intent.

If he pardons himself, Trump will exceed even the worst historical abuses of the Constitution's soul. For those of us who value mercy and principle, a return to the use of the pardon power to manipulate politics and help friends is the worst kind of presidential recidivism. Unfortunately, that might be our sad reality very soon.

Mark Osler is the Robert and Marion Short Professor of Law at the University of St. Thomas and is a former federal prosecutor.