Faced with limited resources, no prosecutor can enforce all of the laws with equal vigor. It is in the discretion of each to decide which to emphasize and which not. The president, as the person in whom the Constitution purports to vest all of the Federal Government’s executive authority, has this discretion within the ambit of the Federal Government.
Note that this discretion doesn’t extend to dispensing with a law altogether, which was regarded by the Revolutionaries as an objectionable practice of the worst British monarchs.
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