Cal. ELEC § 6565

Each group of candidates for delegates that intends to pledge itself to the candidacy of a particular candidate for presidential nominee shall have the endorsement of the candidate for presidential nominee for whom the members of the group have filed a preference. The endorsement of the candidate for presidential nominee shall be filed with the Secretary of State before the circulation of any nomination papers of a group of candidates pledged to the support of his or her candidacy as presidential nominee.