POTUS is putting public pressure on the DOJ to do their job.
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I get your point, but the DOJ is in the EXECUTIVE branch. Trump is the captain of this ship and those people are his “crew”. He is ultimately responsible and accountable for the performance and integrity of the agencies under his purview. That’s the job he signed up for.
There is a political component to this, and the media is against Trump. Do you see the narrative that has been playing out the past week or so, Trump becoming a dictator? Put public pressure on the DOJ to go after Abedin, the DOJ OIG report will serve as the basis to go after the DOJ and FBI officers who conspired to protect Hillary from prosecution and then illegally targeting her political opponent, as well as to re-open the Clinton email scandal which served to protect her pay to play schemes while serving as SOS.