Can declassified information be reclassified?

Question

It seems that the laws around the safekeeping of certain US government and national security information are described by President Obama's Executive Order 13526. These make clear that the ultimate power to declassify documents rests with the executive.

Topically, President Trump appears to be referring to this fact to justify being in possession of "documents with classification markings"; that he had the authority to declassify such during his Presidency and hence the documents are hence no longer subject to the protections demanded of classified documents. This, to a layman, seems like a (nearly) bulletproof defence.

The one exception I can see is if a future President (e.g. President Biden) unilaterally reclassify the document at a higher level, now that President Trump no longer possesses his powers of classification under the Presidential office.

This leads to two related questions:

1. If the classification level of a document has been reduced - so exposing this to a larger audience - can it later be reclassified to a higher level?
2. Can the President or delegated power capriciously classify any document in the possession of a political rival, without him or her knowing, and cause the rival to be in breach of law?

Answer 1

The DOJ has a handy FAQ about declassification on their site. There are statutory and E.O. provisions for automatic declassification of information (I think the most common one in practice is "historically important information after 25 years"). Though this is subject to review and may be rejected in particular situations, dictated by much the same provisions. As for reclassifying automatically declassified information, the FAQ says:

CAN INFORMATION THAT HAS BEEN AUTOMATICALLY DECLASSIFIED BE RECLASSIFIED?

Yes. Information that has not previously been disclosed to the public under proper authority may be classified or reclassified after an agency has received a request for it under the FOIA, Presidential Records Act, 44 USC 2204(c)(1), the Privacy Act of 1974, or the mandatory review provisions of E.O. 13526, only if such classification or reclassification meets the requirements of E.O. 13526, and is accomplished on a document-by-document basis with the personal participation or under the direction of the agency head, the deputy agency head, or the senior agency official. Information may not be reclassified after declassification and release to the public under proper authority.

Similar provisions were detailed in E.O. 12958.

A "proper authority" would not include, say, leaks, like those due to Snowden. Note that, as for Trump's situation, while he most surely counted as a "proper authority" while President, there is no indication whatsoever that the information was also released to the public under his or any other proper authority.

A recent and somewhat controversial example of reclassification began in 1999 under the Clinton administration, as a result of aforementioned E.O. 12958, and lasted for several years after.

To justify their reclassification program, officials at CIA and military agencies have argued that during the implementation of Executive Order 12958, President Clinton's program for bulk declassification of historical federal records, many sensitive intelligence-related documents that remained classified were inadvertently released at NARA, especially in State Department files. Even though researchers had been combing through and copying documents from those collections for years, CIA and other agencies compelled NARA to grant them access to the open files so they could reclassify documents.

A separate article from the same link provides:

...security officials at DOE had become concerned that the implementation of EO 12958 had led to the inadvertent release in State Department and other agency records at NARA of "unmarked" restricted and formerly restricted data on nuclear weapons. In the fall of 1998, Congress formally authorized the Department of Energy to remove from public document repositories any and all sensitive nuclear weapons design-related information pursuant to Section 3161 of the National Defense Authorization Act for Fiscal Year 1999, entitled "Protection Against Inadvertent Release of Restricted Data and Formerly Restricted Data."

The argument the agencies seemed to advance for this reclassification is that prior declassifications had not undergone the proper equity considerations: meaning that agencies other than the ones who declassified them had an interest and say in their classification, which should not have been overlooked. While some complained about the reclassification of information that had entered the public domain—the material had been on the public shelves for years, and some of it had even been published and a good deal more of it sifted through by various parties—this did not seem to matter to the agencies. Much as classified information leaked by Snowden or anyone else remains classified, the information becoming common/public knowledge does not necessarily automatically affect its classification status, or ability to be (re)classified.

Answer 2

Ignoring the question of whether Trump violated US law by being in possession of some document (without concrete details, this cannot be assessed), we can consider what the "laws" are regarding classification of documents and consequences of violating those laws.

"Classifying" information takes place within the executive branch – Congress and the courts do not classify documents. 18 USC § 798(b) defines "classified information" as something "specifically designated by a United States Government Agency for limited or restricted dissemination or distribution", and "unauthorized person"

means any person who, or agency which, is not authorized to receive information of the categories set forth in subsection (a) of this section, by the President, or by the head of a department or agency of the United States Government which is expressly designated by the President to engage in communication intelligence activities for the United States

18 USC 798 makes it a crime to "knowingly and willfully communicates, furnishes, transmits, or otherwise makes available to an unauthorized person" of any classified information. The logic of the classification system depends on someone being always authorized to see and evaluate information in order to deem is "classified" in some sense – POTUS is universally authorized.

There is a regulation that pertains to former presidents, one promulgated by Treasury, saying that

Access to classified information may be granted only to individuals who have a need-to-know the information. This requirement may be waived, however, for individuals who:...Served as President or Vice President.

Note that this addresses (and was device with the intent that) a former president could access classified information which was not in their possession ("access"), and does not bear on the question of whether a current POTUS can legally retain documents which he classified.

There is a separate "whoever" law, 18 USC 793, which says that

Whoever, for the purpose of obtaining information respecting the national defense with intent or reason to believe that the information is to be used to the injury of the United States, or to the advantage of any foreign nation...

does typical spy things like breaking into military facilities, but that isn't relevant to POTUS retaining classified documents.

Anyhow, if other people, who are unauthorized to receive such documents, come to possess them, then there could be consequences. But criminal sanctions under §793 involve (1) illegally entering military / intelligence facilities or (2) authorized persons transmitting such data lawfully in their possession to unauthorized persons. Up to 10 years in prison is the penalty, plus fines.

Here is the currently-visible version of the warrant petition. As you can see, the main allegation is violation is a presidential records retention law (POTUS has to leave his records behind).