Is American democracy so bankrupt that pundits and politicians actively and openly encourage people not to vote for the candidate that most closely represents their interests? Indeed. One would think that in a democracy candidates with diverse political programs would be welcomed--perhaps offering distinct choices on substantive issues. Yet progressives and other leftists are discouraged from voting for their preferred candidates.
In the primaries it was "electability" (which surely the skinny little vegan Kucinich doesn't have, despite consistently offering the most direct and articulate answers in the debates) that should be the key criterion; in the general election it will be the "spoiler" vote--rather than substantive positions--which will be the common refrain against Nader.
But this song is not just the lefty's blues. It concerns everyone who is dissatisfied with the direction of our country, the increasing consolidation of wealth and power at home, and imperialism abroad. Here are two reasons why.
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‘What Americans need is less faith and more thought, less willingness to put their trust in a bygone political order and a greater realization that they, the living, are the only ones capable of maneuvering society through the storm.’<br>Daniel Lazare, The Frozen Republic
US electoral institutions are atypical among industrial democracies
First, the problems of spoiler votes and electability are not unique to recent election cycles or individual candidates. While these issues have been thrown into sharp relief thanks to polarizing effects of the compassionate conservatism of Bush-the-uniter, they are merely symptoms of a broken system. Maybe rigged is a better adjective, since it works very well for the two parties who crafted it to inoculate themselves from third-party competition. In this case "the system" refers to a very specific set of institutions largely peculiar among industrial democracies to the United States.
The two party system, and the winner-take-all voting system (formally called a plurality voting with single member districts) on which it is based, are not mentioned in the constitution. Rather, the institutions of our electoral system take their current form due to concerted action by the two parties and the capitalist interests they both represent. In the presidential election of 1896, William McKinley constructed a vast machine of wealth backed by Northern Industrialists, easily defeating the populist William Jennings Bryan. The populist movement, with roots in the South and West, was effectively crushed. Students of US political history agree that this election was pivotal moment in American politics, laying the foundation for the modern system of electoral politics by ushering in the dominance of wealth in the political system, the growth of non-competitive legislative districts and substantial decreases in voter participation (dropping about 30% in a single generation). A series of reforms ensued--including requirements for periodic registration, voter qualifications, poll taxes, and literacy tests--assuring that poor whites and all blacks would be effectively excluded from participation in the electoral system. Many of these discouraged potential voters never came back to the ballot box. Today, in terms of PACs, business contributes seven times as much as labor, and ten times as much as all other special interest groups.
The results have been stupendous. In the contemporary US, less than half of the eligible population votes in presidential elections and 95% of the population generally does not make significant political contributions. In federal elections, less than one percent of the population contributes more than 80% of contributions over $200.
In effect, there is a "wealth primary" solidly in place, meaning that only those who are able to raise enough money can run--basically, those who will serve the interests of the wealthy. In cases where those who actually oppose the interests of corporate America and neoliberal globalization, and instead represent the interests of working families and global integration with labor and environmental standards, are able to enter the system, such as Nader and Kucinich, then we hear more about electability and spoiler votes than we do of the alternative political programs.
But must we settle for the candidate we dislike least, rather than vote for the one we like most? No. It could be otherwise, as it has been in the United States, and as it is in most other industrial democracies today.
Currently the two major parties receive massive government subsidies, while third parties receive nothing but obstacles. At the legislative level, in a plurality voting single member district (PV-SMD) system like ours each legislative district has only one seat that is won by the candidate who wins the plurality (simply the most, not the majority) of votes. The two parties have the advantage (over third parties) in terms of their entrenched institutionalization in the system, ballot access, nominating procedures, et cetera. In a PV-SMD system, with only one seat per district, the two parties are the only game in town, hence the "spoiler" problem.
An important fact of American history is that the Constitution also does not limit alliances between parties, of a sort that could, even in a PV-SMD system, provide some equivalent to that which in others is typically provided by proportional representation æ to wit, some serious weighting of minority electoral sentiment. Until the end of the 19th century, "fusion" candidacies were universally permitted and very widely practiced. Under their terms, a minor party could nominate the same candidate or slate of candidates as a major one, with votes cast on its line counting toward those candidate(s)' total vis-à-vis rivals. This permitted minor party supporters to vote their values without wasting their votes. By voting on their own line, they could declare their real political identity. By combining their votes with those cast on major party lines, they could stay in the main game. 
After populist defeat in 1896, Republican controlled state legislatures began passing anti-fusion laws as efforts to institutionally solidify the two-party duopoly began in earnest. Over forty states now have anti-fusion laws and only New York routinely uses fusion.
It should also be noted that a PR system allows the political expression of a much wider range of interests. They have also been shown, relative to PV-SMD systems, to increase voter turnout and provide better representation minorities. In a PV-SMD system, when candidates must join either one or the other party to win, there ends up being as much variation within the parties as there is between them. Ultimately, parties become candidate-centered and non-programmatic, which means parties do not have any basic program for which they can be held accountable. Inevitably the distinctions between them become blurred as they compete for the middle. In contrast, a PR system generates parties that have strong programs (and hence are accountable), with a wider range of choices, leading to a truly competitive political system, allowing a diversity of interests to be expressed and represented.