I believe after being elected to the position/office, the POTUS does indeed have total access, it is only when he/she is still seeking candidacy that certain criteria be met (notably: age 35+, natural-born U.S. citizen,etc.).
Sorry, I don’t have any citations at the moment for you, but I do remember that info from grammar school gov’t/civics class.
Actually, for the POTUS, there never has been a formal background investigation. That’s because is was always informally handled by the respective Parties at the urging of Congress.
The only “official” policy I find on Security Clearances in Gov’t, which include employees, are E.O.’s 12958 (Clinton 1995) and 13292 (Bush 2003)
However, these E.O.’s both refer to the Security Policy Board, which Bush abolished in Feb of 2001, which is referred to as National Security Presidential Directive 1.
For members of Congress and their staff...
Atomic energy “Restricted Data” (42 U.S.C. 2162-2168) and “intelligence sources and methods” (50 U.S.C. 403(d)(3)) —
is specified in statute and subsequent rules issued, respectively, by the Department of Energy and Director of National Intelligence.
Other controlled information — such as “sensitive security”
and “sensitive but unclassified” information — is determined largely by executive directives.
Basically, the POTUS establishes a need for a requirement of a National Security Clearance, and delegates to the National Security Board (NSA) to determine the level and the prerequisites.
Congress is actually charged with regulating, by on the record inference, the suitability of the POTUS.