Senate rules state that any nomination must be submitted by an Appointment Message from the legally designated appointing authority for the position in question. The Appointment Message is controlled by the chairperson of the Executive Appointments Committee as the primary and exclusive sponsor. It is then assigned to the Executive Appointments Committee.
At any time, the appointing officer or appointing authority can withdraw an appointment in writing, terminating the legal effect of the initial Appointment Message.
Horwitz's nomination has been assigned to the Executive Appointments Committee, chaired by Sen. Antonio Muñoz (D-1).