California election officials send an “Official Voter Information Guide” to each registered voter in California. The pamphlet contains statements by candidates for statewide office, and also has a page which lists which statewide candidates have been endorsed by which parties. Parties are free to endorse members of other parties.
This year, the June 2014 Guide carries endorsements by the Democratic, Green, Peace & Freedom, and American Independent Parties, but has no endorsements by the Republican Party. This Sacramento Bee article interviews Republican Party state chair to learn why.
The only parties that endorsed candidates from other parties are the Peace & Freedom Party (which endorsed two Greens) and the American Independent Party (which endorsed several Republicans).
There is a controversy as to whether the Guide can carry party endorsements for the non-partisan office of Superintendent of Public Instruction. The Guide sent to Orange County voters includes endorsements for a candidate for that non-partisan office made by the Democratic Party, and an endorsement in that race made by the American Independent Party. The Guide mailed to voters in most other counties do not include those endorsements. Thanks to Mark Seidenberg for that information.