Subject: Re: BLM wants a virtual primary
Do you think that EO would have stuck?

Don't know. Way afield from my line of work.

Gun to my head, it sure looks like the President can do this. The main relevant section of the Civil Service statute (5 USC 7511) provides:

(b)This subchapter does not apply to an employee—
(1)whose appointment is made by and with the advice and consent of the Senate;
(2)whose position has been determined to be of a confidential, policy-determining, policy-making or policy-
advocating character by—
(A)the President for a position that the President has excepted from the competitive service;
(B)the Office of Personnel Management for a position that the Office has excepted from the
competitive service; or

Subsection (b)(2)(A) up there seems to pretty clearly gives the President the authority to except positions from the competitive service and the Civil Service protections generally if he determines the position is "of a confidential, policy-determining, policy-making or policy-advocating character."

Maybe there's a question about whether that can be done retroactively or not, but it seems like this is a power that Congress gave to the President back in the day, and it's just kind of been ignored until Trump really wanted to use it.