Is there any precedent for the Justice Department's investigation into the Russia investigation?

I assume everyone has heard this news by now.

Well, yeah, obviously it’s more kowtowing to the Cheeto-in-chief, but has anything of this sort ever been initiated before; that is, a criminal investigation of the cause of an investigation meant to prevent the country?

I’m not sure what you mean by “prevent the country”, but I would submit OJ’s search for the real killers as a precedent.

The war on whistleblowers is arguably the same thing. Criminal investigations into people exposing criminal behavior.