I believe that casting an informed vote is a civic and religious duty. The problem today is that there is probably not more than 5% of the vote that is informed. Many people will, I think, be cast into hell for, at least partly, casting votes for evil candidates and/or issues (where this applies). I don't, however, believe it is necessarily sinful not to vote especially in such times when the final vote total is virtually meaningless or when there is no acceptable candidate for office.
Take, for example, the U.S. presidential election in 2012. There was no candidate on the ballot or legal write-in candidate in my State (write in votes are not counted unless the write-in is pre-qualified by the State) who was morally acceptable. The popes have allowed the Catholic faithful to, in such circuмstances, to vote for, what we call, "the lesser of two evils", but in no way have the popes ever mandated that we do so.
Even Ron Paul, being the best of all possible candidates in the primary elections, isn't, according to Catholic moral standards, he being a Freemason, an acceptable candidate. No Catholic had the moral duty to cast a vote in the 2013 presidential election.