Prosecutors should select cases “in which the offense is the most flagrant, the public harm the greatest, and the proof the most certain.” But practically speaking, prosecutors have the ability to choose their defendants. And that’s a dangerous power.

With the law books filled with a great assortment of crimes, a prosecutor stands a fair chance of finding at least a technical violation of some act on the part of almost anyone. In such a case, it is not a question of discovering the commission of a crime and then looking for the man who has committed it, it is a question of picking the man and then searching the law books, or putting investigators to work, to pin some offense on him.

He went on to say that “the greatest danger of abuse of prosecuting power lies in” the prosecutor picking “some person whom he dislikes or desires to embarrass, or selects some group of unpopular persons and then looks for an offense.” Here, he said, “law enforcement becomes personal, and the real crime becomes that of being unpopular with the predominant or governing group, being attached to the wrong political views, or being personally obnoxious to or in the way of the prosecutor himself.”

The phrase “law enforcement becomes personal” perfectly captures what’s wrong with Trump’s behavior: He is urging Attorney General William Barr and his counterparts abroad to single out the Biden family for investigation, in the absence of firm evidence, not because a dispassionate process happened to alight on the Biden family, or even because corruption in Ukrainian corporations is high on Trump’s priority list, but because Joe Biden is polling the best against Trump among all the 2020 candidates.

If Joe Biden weren’t a political enemy, this would not be happening. Or as Utah Senator Mitt Romney put it, “When the only American citizen President Trump singles out for China’s investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated.” And remember, Jackson was warning Americans in the 1940s against the dangers of loosing a comparatively liberal U.S. justice system on a citizen, while Trump is urging investigations by the corrupt Ukraine and the repressive, authoritarian China, which are not beholden to any of the safeguards that constrain the Department of Justice.

That is appalling.

Every American who worries about the federal government abusing its power ought to be alarmed by the precedent that Trump is setting. Do you think that he’ll stop at the Bidens? Whom will future Democrats target in this fashion if Trump gets away with this? To accept this abuse of power simply because you’re a Trump supporter is glaringly shortsighted and foolhardy.

Jackson warned:

The prosecutor has more control over life, liberty, and reputation than any other person in America. His discretion is tremendous. He can have citizens investigated and, if he is that kind of person, he can have this done to the tune of public statements and veiled or unveiled intimations. Or the prosecutor may choose a more subtle course and simply have a citizen's friends interviewed. The prosecutor can order arrests, present cases to the grand jury in secret session, and on the basis of his one-sided presentation of the facts, can cause the citizen to be indicted and held for trial.

Trump is abusing the power of investigation. And letting that abuse stand will invite yet more abuse.