When Should the Levee Break?

Short post on a question I've been ruminating on about Trump's prosecution. One of the common arguments I've come across from the MAGA Right is that prosecuting Trump is improper because it's just political retaliation falsely disguised as a neutral and dispassionate application of the law.¹ In support of this argument, you could cite the fact that the apparent mishandling of government records occurs fairly regularly by similarly-positioned politicians (Clinton, Biden, Pence, etc.) and yet its enforcement appears to be selectively doled out. This is potentially also supported by the fact that, speeding tickets notwithstanding, no other US president (former or sitting) has ever been charged for anything before. The fact that US institutions chose to break such a long-standing norm at this particular moment seems a bit too much of a coincidence to believe it was done with honest motivations. [Note: I don’t believe any of the above to be actually persuasive. I’m steelmanning it for the sake of argument.]

Assuming all of the above is true, are there any limiting principles? Until something happens for the first time, it remains by definition "unprecedented", so if your rule is based solely on precedence then nothing would ever be allowed to happen unless it has already happened before which doesn't seem workable. Another consideration also is just because something hasn't happened for a very a long time, it doesn't mean it accidentally created an inviolable precedent that can never be broken now. For example, the crime of piracy is one of the few specifically mentioned in the Constitution and it used to be regularly prosecuted way back in the day but there was a very long lull before the feds dusted it off to go after some Somalis.

I don't think anyone would agree that a permanent bar was created, because that would bestow elected officials and political candidates the extra benefit of potentially perpetual & absolute immunity from all criminal liability, including for conduct that happens after they leave office. In its most absurd implementation, this hypothetical system would allow any criminal a "get out of jail" card just by declaring election candidacy.

So if the longstanding norm against prosecution can indeed be broken, then under which circumstances? For Trump's supporters, I suppose one possible answer is that he has been the target of such a relentless and unprecedented avalanche of (presumably bad faith) lawfare — Russiagate, impeachments, etc. — that trust in the system has been depleted to the point that all action against him should be assumed to be ill-disguised political retaliation as a rule. Assuming that's true, then what? Should the rule be that other politicians can be prosecuted but that Trump should have a carve-out in consideration of the unusually aggressive persecution he had to endure? If so, how serious of a crime would this cover? How long should this immunity last for? Should everyone who faces relentless persecution be afforded similar benefits?

1. I am, of course, assuming for purposes of this post that this is an honest objection rather than an opportunistic one.