MacIntyre's Third Option

In response to my MacIntyre post, Chuck Roth writes:

Here's my question.  I have no doubt that Obama's support of legalized abortion, etc., would require something very grave, indeed, to justify the remote material cooperation which voting for him would entail.  But why does it follow that one should vote for McCain?  McCain is far from perfect; he supports some embryonic stem cell research, some economic policies that I find morally objectionable, and (I think) a misapplication of just war principles. 

One could say that abortion is such a grave thing that it would always (or almost always) constitute a "proportionate reason" to remotely cooperate with the candidate most opposed to it, no matter what other evils one would thereby remotely assist.  But why is a binary choice between Democrat and Republican the only terms of the discussion? I am not nuts, I know that no third party candidate is a realistic possibility this year (or most years); but here in Chicago, there's also no realistic possibility that my vote will be determinative of the election. 

Why must one endorse the lesser evil, if one's endorsement has basically no chance of swinging things one way or another, and if one can avoid endorsing either evil?  Wouldn't a person in a dark blue or dark red state be morally advised to vote for neither Obama nor McCain? 

Admitted, it seems Quixotic and silly.  Go ahead, waste your vote, as they say.  But in another sense, registering one's disapprobation with both parties seems, if multiplied by many thousands, to send a clear message of available votes to the parties, if they alter their positions, or to a potential third party or independent candidate.  Why is that option not discussed in MoJ?  It seems to me a logical slip of great magnitude.