It seems to me that if you disagree, you should reply, not downvote. By replying, you say "I believe there is another way to look at things, and here it is," giving others the opportunity to consider your opinion. By downvoting, you effectively say "because I disagree with you, neither myself nor anyone else should even hear your ideas." And while I sympathize with your "negative presidential vote" idea, I think maybe "none of the above" would be a better solution; if you want to support one candidate over another, you should actually dirty yourself by voting for
that candidate, rather than against the other. Plus, we already have Ralph Nader ;).
About "rehashes": Seems to me it takes relatively little time to either (a) point someone to some of the relevant nodes, or (b) point them to super-search, since you've probably already done the relevant search and/or participated in the relevant discussions to even know that the post is a rehash. Imagine if your high school teachers had, when you asked a question to which the answer was already known, told you to stop talking. What would you have learned from this? Maybe
the poster will follow the links you give, read the other material, and come back with a more informed opinion. Or maybe the poster has already read and digested the relevant material, and doesn't feel the need to cite it.
I find it disturbing to see the root post sitting at -8 now. Are there really 9 people (I gave it a ++) who think that this person's voice shouldn't even be heard, or that there is nothing new to say on the subject of downvoting? I think by even replying you have answered "no".