The committee's chairman, Devin Nunes, a Republican from California, and the Republican majority on the committee have demanded that the Department of Justice turn over documents pertaining to the origins of the investigation of President Trump by special counsel Robert Mueller.

And Nunes has threatened Mueller's superior, Deputy Attorney General Rod Rosenstein, with censure, contempt and even impeachment if he fails to comply. Can Congress interfere in an ongoing federal criminal investigation? Can it get its eyes on law enforcement's active files? In a word: No.  [...]

There are two dangers to the rule of law here. The first is that members of this committee could use their security clearances to examine classified materials and then use what they have seen for a political narrative. They cannot lawfully, except on the floor of Congress, publicly reveal classified documents they have seen, but they can (and they have done so in the past) summarize them publicly -- and with a political spin.

That endangers the sources of criminal investigators, many of whom are people who communicate with investigators at great personal risk and to whom confidentiality has been promised. That confidentiality is recognized in the law as the informant's privilege, and it keeps confidential criminal matters from public and peering congressional eyes until the investigation is concluded.

The second and equally harmful danger is that members of the committee could leak what they have seen. To prevent this, prosecutors have a privilege to keep their files secret until they charge or exonerate their targets or subjects.

Under the Constitution, we enjoy the separation of powers. Congress writes the laws; the executive branch enforces them; and the courts interpret them. Congress can no more constitutionally interfere with ongoing law enforcement for political purposes than the DOJ can interfere with the passage of congressional legislation that it doesn't like.

The down and dirty fear that the DOJ and the FBI have is that revealing the contents of the criminal file on the president to his political allies in Congress in the midst of an investigation of him would be a dangerous precedent, one that would pollute the investigation and give present and future politically powerful potential defendants advantages that no one else has.