That's a common belief (myth?) among people on the left. If everyone has to vote, then they will never lose an election.
It's also why they support and want to continue the two-party system. All they have to do is present themselves as a less shitty version of the Republican candidate and then you HAVE to vote for their corporate benefactor.
I'd like to think that more and more people are seeing through the bullshit and using their "non-vote" as a way to send a message.
http://brilliantmaps.com/did-not-vote/spoiler (click to show/hide)