Cal. Elec. Code § 13

(a) A person shall not be considered a legally qualified candidate for an office, for party nomination for a partisan office, or for nomination to participate in the general election for a voter-nominated office, under the laws of this state unless that person has filed a declaration of candidacy or statement of write-in candidacy with the proper official for the particular election or primary, or is entitled to have his or her name placed on a general election ballot by reason of having been nominated at a primary election, or having been selected to fill a vacancy on the general election ballot as provided in Section 8807, or having been selected as an independent candidate pursuant to Section 8304.
(b) Nothing in this section shall be construed as preventing or prohibiting any qualified voter of this state from casting a ballot for a person by writing the name of that person on the ballot, or from having that ballot counted or tabulated, nor shall this section be construed as preventing or prohibiting a person from standing or campaigning for an elective office by means of a “write-in” campaign. However, nothing in this section shall be construed as an exception to the requirements of Section 15341 or to permit a person to be a write-in candidate contrary to Sections 8600 and 8606.
(c) It is the intent of the Legislature, in enacting this section, to enable the Federal Communications Commission to determine who is a “legally qualified candidate” in this state for the purposes of administering Section 315 of Title 47 of the United States Code.