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 homosexual 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 gay 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.