California Code, Elections Code - ELEC § 2162

(a) No affidavits of registration other than those provided by the Secretary of State to the county elections officials, provisional ballot envelopes that comply with Section 2160, or the national voter registration forms authorized pursuant to the federal National Voter Registration Act of 1993 (52 U.S.C. Sec. 20501 et seq.) shall be used for the registration of voters.

(b) A voter registration card shall not be altered, defaced, or changed in any way, other than by the insertion of a mailing address and the affixing of postage, if mailed, or as otherwise specifically authorized by the Secretary of State, before distribution of the cards.

(c) The affidavit portion of a voter registration card shall not be marked, stamped, or partially or fully completed by a person other than an elector attempting to register to vote or by a person assisting the elector in completing the affidavit at the request of the elector.