California is one of the few states in which the government mails a Voters Pamphlet to all voters, a few weeks before any public election. In partisan primaries, the election law lets qualified political parties furnish an envelope to elections officials, and the elections officials then include that envelope in all the Voters Pamphlets that are going to members of that party. The purpose of the envelope is to make it easy for a voter to send a contribution to that party.
The San Diego Republican Party this year furnished not only an envelope, but a flyer, for insertion into the Voters Pamphlets that are going to Republican registrants. According to this article, the San Diego County Democratic Party sued to stop the insert, but the judge allowed the insert after he ordered some changes in the content of the insert.