I agree with Dahlia Lithwick in Slate: Optically, it was strategically smart to have Rosenstein make the announcement himself. It presents a unified front at the DOJ, and it looks more obstructive if Trump tries to fire him. I just want to elaborate on the quote I provided for her:
As Mueller makes more progress in the investigation, it makes it more difficult legally to fire Mueller because it is more blatantly obstructive. On the other hand, as Mueller gets closer to where the “bodies are buried,” and because Trump may know where the bodies are buried, Trump might take his chances with obstruction charges in order to actually obstruct the revelations that would actually end his presidency. Until Congress passes a veto-proof bill to protect Mueller from firing without cause, what makes Trump think that there would be a significant immediate cost to firing Mueller or Rosenstein? Every indictment raises the stakes of obstruction but also increases the chance of a temper tantrum firing with no clear consequences until Congress changes hands. If Congress can’t pass a Mueller statute, in the very least the Senate intelligence committee needs to make a more public stance.
If Trump fires Mueller, I still think the Senate Intelligence Committee or NY AG Eric Schneiderman could hire him to continue the investigation with subpoena power. But Trump firing Mueller would still delay and disrupt the investigation. Congress needs to act by giving Mueller formal job security: by statute, DOJ officials should not be able to fire Special counsels without good cause. And the President should not have the power to fire special counsels at all.
I’ll have more to come on constitutional and legal arguments for such statutes.