Must a Catholic Vote?

Thomas Peters' blog, The American Papist, links to this CNS article where Most Reverend John McCormack, the Bishop of Manchester, NH explains voting is a moral obligation. Further:

"When candidates support or tolerate policies that include intrinsically evil acts, a Catholic must carefully assess the situation and decide which candidate will produce the least harm to innocent human life, if elected," he added.

I find this a difficult assertion to swallow and fail to see how exercising one's conscience to not materially cooperate with any evil by means of voting, would violate the moral precepts of the Church.

Should Ron Paul, our favored candidate for the Republican Party, not win the nomination, would it be a violation of the moral law should a Catholic chose not to vote for one of the other candidates which could be completely unacceptable? Let's say the ballot consists of Clinton, Giulliani, and the Libertarian candidate. Let's also say that all these candidates remain pro-abortion, pro-unjust war, and pro-death penalty [I know the last category is not necessarily intrinsically evil]. Does this mean that a failure to show up at the ballot box will constitute a sin? I would think that an intentional violation of a moral precept of the Church is a sin. Does such an ommission from voting generally constitute a venial or mortal sin? Does this mean Catholics must vote for and therefore materially cooperate with candidates that formally cooperate with intrinsically evil acts?

Correct me if I am wrong, but I don't think we are ever required by the Church and under Her moral theology to ever materially cooperate with evil. Politicians who explicitly endorse the pro-abortion cause or advance unjust aggressive wars, engage in formal cooperation with such evil acts. Voting for one of these candidates can be a formal cooperation with evil if the voter endorses the candidates' own cooperation or a material cooperation if the voter does not endorse the candidates' views but can find a sufficient justification. So while it can be justified it does not follow that material cooperation with evil is ever required.

Any readers care to give clarification? Should Ron Paul not win the RNC nomination or go on as a 3rd party candidate, I don't see a reason to vote. Are Catholics required to materially cooperate with evil by voting for one of the pro-aborts or pro-unjust warmongers?