Initial Hints from the Op-Ed

The first step in determining authorship is to read the op-ed carefully for what it actually says and doesn't say. The op-ed itself offers several clear clues as to the author’s position in the administration and his (or her) political viewpoint.

Many commentators have carelessly misread the op-ed or not taken the actual text into account when suggesting possible authorship candidates.

Strong libertarian tendencies

He criticizes Trump for not holding “ideals long espoused by conservatives: free minds, free markets and free people.” “Free minds and free markets” is the motto of the libertarian Reason magazine.

He criticizes Trump’s moral lacking in terms commonly used by libertarians and movement conservatives: “The root of the problem is the president’s amorality… he is not moored to any discernible first principles.” “first principles” is a common term used in conservative circles including members of the Federalist Society.

Worked closely with a cabinet secretary from the early days of the administration

 “There were early whispers within the cabinet of invoking the 25th Amendment” suggests that the author heard those whispers at the time and was in a position to hear them.

“No one wanted to precipitate a constitutional crisis. So we [emphasis added] will do what we can to steer the administration in the right direction until — one way or another — it’s over” implies that the author is working with a cabinet secretary who has the power to invoke the 25^th amendment but has chosen not to.

Not involved in national security.

“His national security team knew better” implies that the author is not part of the national security team.

Someone who was involved in national security would not write this anonymous op-ed. It would put that person in a compromised position, having a secret that could be used against them.

Well-connected mainstream DC Republican, not an outsider 

He quotes other officials and suggests he is in contact with people across the administration: “the White House to executive branch departments and agencies, senior officials will privately admit their daily disbelief at the commander in chief’s comments and actions”. A longtime Washington insider would have that kind of access and connections.

He praises Trump’s nemesis and mainstream Republican hero John McCain as the parting shot of the op-ed.

Has nothing to lose if his identity is divulged

Whoever wrote the op-ed is obviously a very meticulous and careful individual. He must have known that his identity could easily be revealed. This person must have a secure livelihood that doesn't depend on his success in this job or the support or the approval of the Trump administration. He must be someone who can "take or leave" his job in the administration.