I've voted Dem all my life (since 1988), and while my preferred candidate has won several of those races, my actual VOTE never helped them because I voted in Mississippi (88), Alabama (92), and Texas (96 & thereafter) -- all of which have been GOP strongholds for a long, long time. (Texas, for example, hasn't gone for the Democrats since Carter v. Ford in 1976.)
It's easy to imagine that a feeling of despair about the efficacy of one's vote would drive someone to stay home.