Trump is thinking about invoking an ethics rule to keep Mueller from investigating Jared Kushner’s Russian connections. https://www.vox.com/2017/5/20/15668162/kushner-trump-russia-corruption
I think Trump and Kushner come off looking unreasonably obstructionist (read guilty) if they push this one. Ethics rules relating to an attorney taking a role against a former client run through the entire practice of law, not just government. An attorney cannot sue A for B, and later sue B for C, if both suits share overlapping legal or factual issues. It has to do with the duty of confidentiality an attorney owes to a client; the assumption is the attorney would have knowledge of the former client’s legal strategies and information learned as part of the attorney-client relationship. It would be a conflict of interest with the new client not to use such information and a conflict with the old client to use it, so the attorney is barred from taking the new case.
Technically, every attorney in a law form is considered to be associated with the representation of all the firm’s clients. But if the attorney had nothing to do with the prior representation of a client by the attorney’s former law firm, the attorney would not know any confidential information. So there are generally rules for determining that confidential information is not an issue so there is no ethical reason an attorney cannot take on the new case. This would be especially likely when the attorney’s former firm was really big. Muller’s firm, WilmerHale, has over a thousand attorneys.
Given that the ethics rule in question is a federal rule applying to federal prosecutors, I think it is safe to assume its application is thoroughly bureaucratized. There are likely extensive memos on when a waiver should be given as well as complete records of when DOJ has given or denied such waivers and why things were decided the way theywere. If Mueller’s work at WilmerHale had absolutely nothing to do with Kushner, these DOJ rules and historical records would establish Mueller would normally get a waiver. It isn’t like Trump sitting in the Oval Office and flipping a coin or using his gut to make the decision. The denial of a waiver because Trump interfered, would blow up in his face just like firing Comey did. Even trying to meddle makes it look like something incriminating is being hidden.