See post 149. As Revel points out any argument which states a non-vote for Romney is a vote for Obama is exactly mirrored by the argument that a non-vote for Obama is a vote for Romney. I am a total ABOR (anyone but Obama or Romney). Just because I'm more opposed to Obama, it doesn't follow that my vote is available to Romney.
To your example:
Flaw #1: I am not compelled to vote. It is perfectly within my Biblical and Constitutional rights to abstain if I so chose.
Flaw #2 (also known as the category fallacy): Your example assumes that I can be persuaded to vote for Romney, in other words, I can be moved from one category to another. (This is also crudely referred to as the "if my aunt had testicles, she'd be my uncle" fallacy.) Since I am firmly opposed to Romney, it could be argued that I could be just as easily persuaded to vote for Obama since I rather fight the enemy outside the gates than an enemy inside the gates (but I won't). See post 149.
Flaw #3 (also known as the proportions fallacy): Your example puts too much importance on one act which distorts the reality of the situation. My vote (or yours) means basically nothing in the overall scheme of things. See post 191. My state went for McCain by about 186K votes (or 15 percentage points). For Obama to carry this state, far more 2008 McCain voters than just me are going to have to decide that Romney is a bridge too far.
Flaw #4: Your example is a popular vote example. While the popular vote correlates to some degree, it is the electoral college which decides the Presidential race. In this regard, your example is irrelevant. Practically speaking, if Romney needs my vote to carry my state, he's already on the losing side of a historical landslide.
By your logic, I'm going to be contributing to the Obama campaign as well because I'm not giving any money to the Romney campaign. I'm sure you'll balk at this but think about it: giving my money to a campaign is as voluntary an act as my selection of whom I'm going to vote for. A non-vote for Obama and Romney is not a vote for either man just as not giving money to either man is a lack of financial support for both men.
If only you had been in charge of the Dem's GOTV in 2008, we wouldn't have suffered under Obama these past three years....