Here’s how the Justice Department failed to follow the rule. As part of the scramble in the executive branch caused by the whistle-blower’s complaint, the Justice Department secretly investigated Mr. Trump for a potential campaign-finance violation. The department reportedly cleared him because the contributions solicited from a foreign government to his campaign were not quantifiable “things of value.” That’s the key phrase in one of the most important campaign-finance laws…

For over 40 years, a memorandum of understanding has stood between the Justice Department and the Federal Election Commission, and it has been duly entered into The Federal Register. It’s a guide for how Justice and the F.E.C. should interact in administering federal election law. The document recognizes that some election law violations, for whatever reason, “may not be proper subjects for prosecution as crimes” under key criminal provisions of the federal election law statutes. The document then explains how the two agencies should interact when one or the other learns of potential violations.

Here’s the key part for our purpose: When information comes to the attention of the Justice Department indicating a “probable violation” of the Federal Election Campaign Act, the document says, “the department will apprise the commission of such information at the earliest opportunity.”