Not to say they are all bad. These mailers are essential for candidates to reach a large number of voters without spending big money! Check out the mailer understand they are put out by special interests in some cases. Just do your homework.
From the LAGOP:
One cautionary general comment—As ballots have been mailed out, we are becoming inundated by slate mailers—BE WARY!! One official-looking mailer has the picture of Ronald Reagan and the mailer made it appear as though the Republican Party was advocating a Yes on Prop 13, as well as supporting a number of Democrat candidates. Don’t be fooled!! The LAGOP is NOT sending out slate mailers of any kind this election cycle. All our LAGOP endorsements are located on our website. For local races, contact our office at firstname.lastname@example.org and we will connect you with your local Republican Assembly District Committee to obtain more specific localized information.