Leave it to a brilliant lawyer to make a brilliant point. Harvard Law professor and Donald Trump supporter Alan Dershowitz points out the Deputy Attorney General Rod Rosenstein cannot continue to serve in his government job if Special Prosecutor Robert Mueller accuses President Trump of obstruction of justice in connection with the firing of James Comey as FBI director last year.
Dershowitz notes that Rosenstein was deeply involved in the firing and drafted a memo for the president outlining the grounds for the dismissal. He says that Rosenstein cannot both be a witness and the supervisor of the prosecution. Great point!
As Mueller rummages around Trump's attorney's office looking for a crime with which to charge the president, it is increasingly likely that a substantive crime like coordination with Russian intelligence to fix the election is unlikely to be provable. That probably means that a procedural crime relating to the investigation itself is the most likely charge Mueller will be able to bring against the president. Obstruction of justice would likely be the accusation.
But if Rosenstein is too conflicted to continue his government service (I would say "public service, but he hasn't done much of that), he must be replaced.
His replacement, of course, would then be in charge of the prosecution of the president, a power Rosenstein now has by virtue of Attorney General Jeff Sessions' recusal from the investigation. So, Mueller would gain a witness but acquire a new boss.
Mueller cannot bring obstruction charges without questioning Rosenstein as a witness, precluding the deputy attorney general from remaining in office.
This excellent point goes to show why the president needs to bring Alan Dershowitz onto his legal team. Dershowitz knows how to combine excellent legal research with skilled courtroom advocacy and articulate public commentary. Trump, devoid of proficient counsel, needs to bring in Dershowitz.