Has a C-130 ever been designated as "Air Force One"?
As I am sure you are aware, the convention is that any aircraft that has the current US president on board is given the "One" call sign.

For military aircraft, this is the branch of the military followed by "One", which is why you have Air Force One, Marine One, Navy One. If he is on a civilian jet, it is called Executive One.

The actual plane that everyone calls Air Force One is called VC-25, which is the military version of the 747.

There have been many planes that have been called Air Force One, from a King Air B90 to Gulfstream jets and even a combat aircraft like the S3 Viking (which was called Navy One).

However, as far as I know - no president has flown in a C-130 while in office.

Just to close this off, if a president is no longer sitting and is transported; then the callsign changes to SAM (Special Air Mission); so for the "Air Force One" 747, this would be SAM 28000 or SAM 29000 - depending on which of the two is being used.

The classified callsign for Air Force One is Angel.