the “not voting for X is a vote for Y” is a great emotion grabber but it is a fallacy, for not voting for X creates no additional vote for Y
The other day, I had a conservation with an ABO who has become an aggressive Mittbot. After hearing the same old lecture on how the Romney non-vote was a vote for Obama and insisting that I must vote for Romney, I told him not to worry - I was voting for Romney - by not voting for Obama. He paused for a second as he thought about what I said, laughed, and told me that he understood my point. I expect that I won't hear that foolish from him again.