Traditionally, the protection against improper prosecution of citizens has been the judiciary. However, slowly, over the years, the DOJ has tried to exert an influence over the federal judiciary itself.
When a judge is appointed, it is the DOJ who investigates and presents the findings to the Senate. They even make a formal and separate recommendation for fitness. Recently, Congress removed a federal judge from office by impeachment articles (among) which charged he denied in his FBI investigation “..that there was anything in his background that could compromise him, embarrass him or the president if it became publicly known.” But in the opinion of the FBI that wasn’t true. With this as a tool, the DOJ could cause the removal of any judge now on the bench or, at least, embarrass them. All they have to show is that there was some past behavior that is embarrassing to the judge and/or the president. So the test no longer is just “high crimes and misdemeanors”, but only that some unrevealed past behavior was embarrassing. That is some awful power to hold over the federal judiciary.