Every once in a while, I poke around in my spam filter in my gmail accounts.  I virtually never find anything, but today I did.  3 emails from gopsenator.com in one account.  Three from nrccmail.org, and some other gopsenator.com in the other.  All fund-raising emails.  If potential donors (sorry senators, I'm not a donor) don't get the emails, then funds are decreased.
So, built in bias?  Lots of clever dems subscribing then marking as spam?  Who knows.  I'll do my part by un-marking them as spam and deleting them manually.