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U.S. President Joe Biden waves as he arrives on Air Force One at Delaware Air National Guard Base in New Castle, Del., on Jan. 15.

Carolyn Kaster/The Associated Press

The curious, bewildering, ever-deepening case of the secret, and secreted, presidential documents has upended the established behaviour patterns of Washington.

With new confidential documents continuing to turn up, the issue of improper possession of secret materials has moved from a Donald Trump scandal to one shared by him and his successor, Joe Biden. The Justice Department decision about whether to charge Mr. Trump with unlawful possession of government documents has spawned a parallel decision over legal action against a president of the attorney-general’s own party.

The Republicans who ordinarily might be gleeful about the prospects of a special counsel examining the conduct of a Democratic president are instead troubled that the investigation might overshadow, or neuter, their own probes.

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Above all, the unusual phenomenon of two presidents – possible opponents in next year’s election – sharing the same potential crime could undermine the possibility that Mr. Trump faces federal legal action at all.

From the start, it has been clear that amid all the legal investigations into Mr. Trump’s business, personal and political activities – including his efforts to overturn the 2020 election and his role in fomenting the Jan. 6, 2021, insurrection at the Capitol – the one that had the best chance of initiating federal charges involved his possession of secret documents in his Mar-a-Lago home.

The reason: The potential documents case does not involve prosecuting a former president for his actions while in office; his alleged crime occurred after he left the White House, as separate from any claims about presidential immunity – itself a specious concept, subject to debate and court rulings – as a traffic violation he might incur as an ex-president driving in Palm Beach.

In many of the other areas of legal vulnerability, Mr. Trump has the possible (but not necessarily successful) option of claiming executive privilege, a murky principle that presidents have clung to since the Richard Nixon years. There is no plausible avenue of employing executive privilege in the documents case.

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Probes into Biden, Trump’s handling of documents prompt questions about what consequences may await them

But now that Mr. Biden has admitted that he possessed secret documents in violation of federal law, the power of the charges against Mr. Trump has been substantially weakened. It is not so much a case of “everybody does it” – clearly no other president has done such a thing – as it is of the proverbial sauce for both goose (Mr. Trump) and gander (Mr. Biden).

The White House sees it differently, of course. The Biden team argues the President volunteered his possession of the documents while Mr. Trump did not. It argues Mr. Biden didn’t fight federal authorities in an effort to turn over the documents while Mr. Trump did. All that may be true, in point of fact. None of it matters, in point of politics.

Both presidents now are subject to special counsels who will make recommendations to Attorney-General Merrick Garland. The two counsels are not co-ordinating their investigations. They will issue separate findings to the Justice Department. Mr. Garland will make separate decisions on the two matters.

“In the public mind these cases may be connected,” said Margaret Russell of the Santa Clara University School of Law. “But they will examine entirely different things. They’re not working in offices next to each other and not consulting each other – and they can make entirely different recommendations.”

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But how separate can Mr. Garland’s decisions be? The differences between the two cases – predictably, Democrats insist they are manifold, Republicans argue they are insignificant – almost certainly will vanish in the public mind.

Fairness, or at least perceived fairness, will make it exceedingly difficult for Mr. Garland to proceed with federal charges against one without doing so against the other. It won’t matter whether one president’s documents are more sensitive than the other’s, or whether one had more documents than the other, or even whether one sought to obstruct justice while the other did not.

There are several recent examples of court cases against officials who mishandled classified information. One went to prison. Another was former CIA director John Deutch, who was pardoned by then-president Bill Clinton. In 2003, Mr. Clinton’s one-time national security adviser, Sandy Berger, removed classified documents from the National Archives and was sentenced to 100 hours of community service and probation, fined US$50,000, lost his law licence and was assessed US$6,905 for administrative costs.

The complications don’t end there. The new Republican majority in the House of Representatives is readying a series of investigations into Mr. Biden. Now leading conservatives are troubled that the probe by special counsel Robert Hur might impede their own investigations, by permitting the White House to frustrate their probes by shunting off demands and subpoenas with the claim that the matters are “under investigation by a special counsel.”

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They also worry Mr. Hur might expand his inquiry, thus constricting the range of matters House investigators might examine. The last high-profile special counsel, Kenneth Starr, expanded his investigation far beyond its original remit, an examination of Mr. Clinton’s involvement in the Whitewater real estate project, eventually looking into his sexual relations with a White House intern.

Last week, in an interview with Fox Business, GOP Representative James R. Comer of Kentucky, the new head of the powerful House Oversight Committee, warned that Mr. Hur “better be just focused on Biden’s handling of classified documents.” The implicit notion: Other areas of pursuit should be left to his committee.

In the end, the entire episode might come down to an exchange in an 1845 story by the American writer Edgar Allan Poe entitled, poignantly, The Purloined Letter.

“The fact is, we have all been a good deal puzzled because the affair is so simple, and yet baffles us altogether,” one of Poe’s characters says, to which the other responds, “Perhaps it is the very simplicity of the thing which puts you at fault.”