Illinois constitution
Article V
Section 7

If the Attorney General, Secretary of State, Comptroller or Treasurer fails to qualify or if his office becomes
vacant, the Governor shall fill the office by appointment.
The appointee shall hold office until the elected officer
qualifies or until a successor is elected and qualified as
may be provided by law and shall not be subject to removal by
the Governor. If the Lieutenant Governor fails to qualify or
if his office becomes vacant, it shall remain vacant until
the end of the term.