In reviewing the precedents, Mr. Mueller could have concluded narrowly — and rightly — that the Trump Tower meeting by itself rightly was not a prohibited “thing of value” and did not appear to be “opposition research.” However, when he later discussed “opposition research,” he should have clarified that even if such a category is hard to define, many kinds of research clearly cross the line.

Instead, he puzzlingly made a giant leap to imply that most opposition research — even by foreign governments — might be covered by the First Amendment. He conflated a valid point with an invalid point: The language of “anything of value” is indeed vague and arguably creates a chilling effect, and it should be applied carefully. Merely sharing readily available information must be protected speech. Nevertheless, information that is clearly “opposition research” as a significant spending of time and resources to oppose and to benefit candidates is not constitutionally protected, because the government has a compelling interest in limiting such corruption.

That legal error has serious consequences. First, just as Mr. Trump’s comments were an invitation to foreign governments to help, the Mueller report is an unwitting invitation to Mr. Trump and foreign governments to use opposition research to evade campaign finance law. Second, if any prosecutor tries to indict Mr. Trump or any candidate for trying to use this “opposition research” loophole, defense counsel will cite the Mueller report as a solid defense. Prosecutions for campaign finance violations require proof that “a violation must have been committed knowingly and willfully,” that the defendant knew he or she was breaking the law. But now independent groups or foreign governments can spend millions to provide campaigns with opposition research files, then quote the Mueller report and say they were simply “recounting accurate facts.” Campaign officials can offer the same defense.

The report’s other major error was on the law of campaign coordination. The Justice Department’s initial appointment explicitly tasked Mr. Mueller with investigating campaign “coordination.” His report stated that “‘coordination’ does not have a settled definition in federal criminal law. We understood coordination to require an agreement — tacit or express …” To the contrary, Congress declared in 2002 that campaign finance regulations “shall not require agreement or formal collaboration to establish coordination.” The F.E.C.’s regulations state: “Coordinated means made in cooperation, consultation or concert with, or at the request or suggestion of, a candidate,” with no need to show any kind of agreement.

The report found sufficient facts to find impermissible coordination, but his failure to say so, along with his incorrect statement on the law, opened another invitation to evasion.

Remarkably, Mr. Mueller showed more deference to a White House Office of Legal Counsel memo (on not indicting a sitting president) than to Congress and the F.E.C. (on campaign finance law). Members of Congress should press him on these backward assumptions.

A prosecutor’s duty is to explain the law clearly and correctly — not only to the subjects of the investigation but also to other candidates and to the public. This duty now falls to Congress and the F.E.C. Good-faith candidates deserve fair notice of where the line is, and bad-faith candidates need to be deterred by clarity.