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There's not a blanket grave moral obligation to vote.  That's not really how positive precepts typically work.  That said, I think that sometimes there is.  Fr. Stephen Webber, one of my favorite priests and one who I'm sure everyone (who's met him) knows is of the Williamsonite formation and by no means a laxist or one who strives for human respect, convinced me that there was a moral obligation to vote against ɧoɱosɛҳųαƖ marriage (this is consistent with the material quoted just above from Ladislaus).

Now that of course supposes the system actually works (i.e. that your votes are counted and so on).  If that weren't the case then there wouldn't even be a moral obligation to vote against something like gαy marriage, I don't think.  So any obligation of course presupposes that the system works the way it says it does.  

In terms of a moral obligation to (say) vote for Mitt Romney against Obama, I'm incredulous.  When it's a "lesser of two evils" decision, I think that the idea of selecting the lesser is conditioned on there only actually being two options; but there's at least one other option: don't vote!  Or vote for Christ the King, or whomever.  And voting for the lesser of two evils isn't analogous to voting for the sanctity of marriage, since voting for the sanctity of marriage is an intrinsic good, while voting for a neocon isn't.

This is only true if your vote is only affecting politics.  Nowadays, politics and sin are intertwined (i.e. abortion) so you do have a moral imperative to vote.
Many politicians are clearly pro-abortion, so the obligation would be to vote against all openly pro-sin candidates.

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Well I would grant that in a case where a politician is actually anti-abortion.  But many "conservative" politicians wouldn't move to outlaw abortion, only to redefine its acceptability (but they do so short of saying it is never acceptable).  I don't have a moral obligation to vote for a man who is OK killing a thousand babies when the opposition is OK killing ten thousand.
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That isn't to say that it might not be prudentially wise to vote for a candidate who (say) wants to supremely limit abortions (even if not outlaw them) if they're up against someone who wants to completely open up the abortion industry.  I'm only talking about the existence (or lackthereof) of a moral obligation, and that doesn't exist for any politician who would not vote to outlaw abortion entirely.

There is nothing "inherently anti-Catholic" about the concept of the lesser evil nor about the language in which the concept is expressed, certainly not in the cited context.

Oh, there most certainly is, claudel.  For all your pseudo-intellectual blustering, you have no idea what you're talking about.

You have absolutely no substantial argument to make but resort to a cheap ad hominem -- "One needs to be pretty dense—or else very malicious—to assert that the concept is equatable with an inversion of the moral order. "

People usually resort to this when it's all they've got.  I'll rebut this with my equally gratuitous assertion, but in much less flowery language:  You're an idiot.