California Code, Elections Code - ELEC § 359.5

(a) “Voter-nominated office” means a congressional or state elective office for which a candidate may choose to have his or her party preference or lack of party preference indicated upon the ballot. A political party or party central committee shall not nominate a candidate at a state-conducted primary election for a voter-nominated office. The primary conducted for a voter-nominated office does not serve to determine the nominees of a political party but serves to winnow the candidates for the general election to the candidates receiving the highest or second highest number of votes cast at the primary election. The following offices are voter-nominated offices:

(b) This section does not prohibit a political party or party central committee from endorsing, supporting, or opposing a candidate for an office listed in subdivision (a).