It's not the voters who are the problem. It's the candidates.
Not the Republican candidates - the GOP is running the reddest of red meat conservatives, and they seem to have found the right balance of crazy this time around. Less "I'm not a witch", more "I like castrating hogs." They consistently activate their base and get their voters to turn out.
Meanwhile, the Democratic Party won't field candidates that will motivate their base. Nearly every Democratic candidate this cycle refused to campaign with President Obama, and they seemed to be in a contest to see who could turn the furthest away from him without actually calling for impeachment themselves. They refused to run on the success of their own President from their own goddamn party. How the hell do you convince someone to vote for a Democrat when you run away from everything the Democratic Party and its President has accomplished in the past six years?
Even more, how do you not talk all day and night about how this was all accomplished in the face of overwhelming obstruction and by your Republican opponent and their party? How do you not hang their shutdown of the government around their necks? Why do you refuse to highlight how they have used the little power then have for partisan gain, and consistently opposed policies that are favored by vast majorities of people?
It's not the voters who are stupid - they know what they want, and if they get a candidate who advocates for those things they will come out and vote. See Obama '08 for example. But if the Democrats keep running candidates who refuse to stand up for progressive values, they will stay home, and the Republicans will win because their voters show up.