Many candidates pay to be included on targeted slate mailers. Sometimes they are deceptive, like when a Republican Simi Valley City Council Member pays to be included on a mailer with the headline: Voter Information Guide for Democrats. That's okay as she helped to fund a mailer for all of the Democrats listed that did not pay!
If there is an asterisk next to a candidate's name that means they paid to be included on the slate mailer. The best I have seen is when conservative
Democratic Party Republican Party leader Audra Strickland paid to be on a mailer with John F. Kennedy on it. Audra Strickland is a proud conservative Republican. Her inclusion on that mailer doesn't reflect her voting record.
The endorsement of a party allows access to resources for a candidate including databases of voters, inclusion on official slate mailers, invitations to events, being put on the agenda to speak to party club meetings, and more.
Because of the great campaigns being put on by many proud Democrats like Nathan Sweet, Bob Rust, Marie Panec, and more I expect that Democrats will come out as winners in Ventura County this week. They won't win every contest but they will have a deeper bench of leaders after the ballots are counted.