Jacob Leibenluft, in his article in Slate, has missed an important point. To understand the pardon power, we need to examine just what is happening when an executive with pardon power grants a pardon. What he is saying, essentially, is “I won’t enforce a sentence against x for y, and I bind my successors not to do so as well.”
Where the question gets interesting is when we ask if he can grant a pardon for a conviction that has not yet occurred, or prevent a trial from being held. From my historical research, and despite Ex parte Garland, I find the answer to both is no. A pardon has to specify a sentence as well as the defendant, and that can’t be known before conviction. Granting a pardon to someone for anything he might be convicted of, in advance of such conviction, is in conflict with the constitutional prohibition against granting titles of nobility, and exempting someone from prosecution for anything at all is making that person a noble, even if it comes only with a title of “he who is above the law”. Leaving aside the obvious likelihood that the Court in Ex parte Garland was corrupt, this point was not argued before the Court and therefore the precedent does not cover it.
Even if we ignore the problem of conflict with the title of nobility prohibition, it cannot be logically inferred from the pardon power that a pardon can prevent prosecution. The president may refuse to carry out a sentence but he has no power to prevent a charge from being filed, an indictment obtained, and the court from trying the accused. The court might be reluctant to do so if the sentence won’t be imposed, but a trial serves many purposes besides executable conviction, one of the most important of which is to bring out the truth, and it may be important to proceed with trial even if the conviction won’t be executed.
There is also an issue of whether a president can bind his successors not to enforce a conviction. That is an implied power of a monarch, but not of a president. My finding is that the pardon power of the president is not the power to bind his successors.