In 1940, one year before he became a Supreme Court justice, then-Attorney General Robert H. Jackson addressed a conference of United States Attorneys and delivered a short speech titled “The Federal Prosecutor” that is regarded as a classic statement of the duties of a federal prosecutor.

1. Jackson recognized that the exercise of prosecutorial discretion—the prosecutor’s picking which cases to pursue—is both necessary and dangerous. The exercise of prosecutorial discretion is necessary because no prosecutor could conceivably prosecute every violation:

If the Department of Justice were to make even a pretense of reaching every probable violation of federal law, ten times its present staff would be inadequate. We know that no local police force can strictly enforce the traffic laws, or it would arrest half the driving population on any given morning. What every prosecutor is practically required to do is to select the cases for prosecution and to select those in which the offense is the most flagrant, the public harm the greatest, and the proof the most certain.

The exercise of prosecutorial discretion is dangerous because it enables a prosecutor to “pick[] some person whom he dislikes or desires to embarrass” while leaving alone other persons whom he likes or wants to curry favor with.

2. But it’s noteworthy that Jackson stated a very different view of prosecutorial discretion on laws involving national security:

In the enforcement of laws which protect our national integrity and existence, we should prosecute any and every act of violation….* [Emphasis added/altered.]

To be sure, the prosecutor’s judgment is always involved in determining whether there has in fact been a violation (and whether there is sufficient admissible evidence to prove such violation). But under Jackson’s view (at least as expressed in this speech), Comey should have limited himself to determining whether Hillary Clinton violated federal laws governing handling of classified national-security information. He should not, in other words, have recommended against prosecution based on his assessment of what had been involved in prosecutions against other defendants under those laws.

* In context, Jackson is emphasizing the distinction between prosecuting unlawful acts and persecuting unpopular opinions or other exercises of civil liberties. But careful wordsmith that he was, his use of “any and every act of violation” (rather than, say, “only acts of violation”) was surely deliberate.