The first reason why voting is important related to ethics. Many people have put their lives on the line, and have been maimed and killed in order to preserve the basic fundamental liberties we enjoy today. These sacrifices of precious lives have been made in several wars in which America has fought, as well as various social movements such as women's suffrage and the civil rights movement. Among the most important of the liberties that were preserved by these sacrifices is the right to have a voice in one's own government. Whether we like it or not, those sacrifices obligate us on an ethical and moral level to make sure they were not made in vain. The right to vote is a gift from them; and it would be in very bad form to throw that gift away considering its cost.
The second reason why voting is important is because it is a prerequisite to legitimate expression of discontent with the performance and direction of the government. A person who was able to vote but did not loses any credible force of opinion when things are going badly. This is because they chose to abdicate their ultimate opportunity to exercise their voice in government. It becomes a credibility issue. For example, a person who failed to vote one way or the other in the 2004 Presidential Election should not complain about the current state of the economy or the loss of lives in Iraq. Even those who voted for Bush have the right to complain about the way things are going because they utilized their opportunity to express their voice in the government. Any easy way to redeem that right to complain would simply be to vote in the next election.
Finally, the American system of government depends upon the peoples' involvement in the political process through voting. If there were a mass failure to vote, it would threaten the very nature of republican democracy that is the essence of American government. Thus, Americans have a responsibility to their country and their way of life to make sure they vote. 20% voter turnout is dangerous, because it enables a small fraction of the people to determine the composition and direction of the government. At some point, there may be a very real concern about the democratic legitimacy of the government. The obligation of every American is to vote to ensure that such legitimacy comes through loud and clear.
Voting is perhaps the most important civic duty in America. It is both a right and a privilege that was purchased with the blood of countless Americans. Thus, the responsibility to vote is as much a moral obligation as it is a civic responsibility. Those who fail to exercise their vote relinquish the right to credibly express discontent regarding state of public affairs. Furthermore, because voting is the very foundation of American government, American citizens have a responsibility to ensure government legitimacy by turning out to vote in strong numbers. The public apathy that has resulted in poor voter turnout in several election is a dangerous development for America. People must be educated about the importance of enfranchisement in their every day lives as Americans so they do not take for granted that which indifference could easily take