Fallacy:
"You have a duty to vote!" "If you don't vote, you have no right to complain!" "If you don't vote, your silence is supporting the incumbent!"
Response:
[[http://youtu.be/6dqsNrmXgP0|Here] is a good response from George Carlin.
In a more serious vein, consider the options you face when you vote: If you are in the majority, you are imposing your candidate upon the minority. You are forcing those who disagree with you to accept your preferences, and government edicts are always enforced at gunpoint. When you are trying to choose the lesser of two evils, you are still choosing evil. Lastly, there can be no duty imposed upon you by another party in a free society. If someone can impose a duty upon you without your consent, you are a slave.