Cal. ELEC § 2151

(a) At the time of registering and of transferring registration, an elector may disclose the name of the political party that he or she prefers. The name of that political party shall be stated in the affidavit of registration and the index.

(b) (1) The voter registration card shall inform the affiant that an elector may decline to disclose a political party preference, but a person shall not be entitled to vote the ballot of a political party at a primary election for President of the United States or for a party committee unless he or she has disclosed the name of the party that he or she prefers or unless he or she has declined to disclose a party preference and the political party, by party rule duly noticed to the Secretary of State, authorizes a person who has declined to disclose a party preference to vote the ballot of that political party. The voter registration card shall further inform the affiant that a registered voter may vote for any candidate at a primary election for state elective office or congressional office, regardless of the disclosed party preference of the registrant or the candidate seeking that office or the refusal of the registrant or candidate to disclose a party preference.

(2) The voter registration card shall include a listing of all qualified political parties. As part of that listing, the voter registration card shall also contain an option that permits the affiant to decline to disclose a party preference. This option shall be placed at the end of the listing of qualified political parties.

(c) A person shall not be permitted to vote the ballot of a party or for delegates to the convention of a party other than the party disclosed as preferred in his or her registration, except as provided by Section 2152 or unless he or she has declined to disclose a party preference and the party, by party rule duly noticed to the Secretary of State, authorizes a person who has declined to disclose a party preference to vote the party ballot or for delegates to the party convention.

(d) As of the effective date of the statute that added this subdivision, any voter who previously stated a political party affiliation when registering to vote shall be deemed to have disclosed that same party as his or her political party preference unless the voter files a new affidavit of registration disclosing a different political party preference or no political party preference. Any voter who previously declined to state a party affiliation shall be deemed to have declined to disclose a party preference unless the voter files a new affidavit of registration disclosing a different political party preference.

(e) The Secretary of State may continue to supply existing affidavits of registration prior to printing new or revised forms that reflect the changes required pursuant to any amendment made to this section.