OK, I thought of one more excuse that the 1/3 and 1/4 voters will make; "we won't participate in a corrupt system; the system must change".  Well, I've got news for you, it's unlikely that the system is going to get any better or less corrupt with another Republican administration. 

Actually, when I think about it, there isn't one single good (enough) defense of liberals acting against their own self interests by voting for McCain.