This sort of "lesser evilism," refraining from voting for the best candidate because doing so might inadvertently elect the worst candidate, does more to destroy the moral fabric and political viability of a democracy than any real or imagined evil that might be achieved through the electoral victory of whomever we imagine to be the "bad guy" beneficiary of voting our conscience.
Here are some reasons why:
First, "lesser evilism" is a moral and spiritual slippery slope. We start by giving our vote to a candidate who supports and is a product of a social reality that we actually deplore, and we end up learning to accommodate ourselves to moral corruption in other aspects of our lives. Just as "lesser evilism" teaches us to accommodate to "reality" in politics, so we accommodate to the reality of our economic marketplace, with its ethos of materialism and selfishness. Since everyone else is "looking out for number one," we learn that the way to "make it" is to go along with a set of practices that involve cheating or hurting others in our pursuit of success, making environmentally destructive or morally insensitive choices, and using the excuse that we must focus on "the bottom line" and not on the fine points of moral behavior.
To the extent that we come to believe that we have no alternative but to accept the lesser evil, we lose the inner quality of soul that makes it possible to fight for anything against the odds. We forget how to stand up for our own ideals, and soon we don't see the point in even thinking about what kind of a world we really believe in ("It's so unrealistic"). Internally, we may feel cynical about the world we live in, but as long as we've adopted the attitude that we can't really fight it and must accept its terms, we have cast our vote in favor of keeping what is. Moral courage and hope begin to feel like anachronistic concepts.
Not surprisingly, as people become used to making this choice in daily life, they become most angry not at the forces of evil they accommodate but at those who retain their commitment to fight for their highest ideal. Thus, rage in liberal circles at Nader supporters or in conservative circles at Buchanan supporters--both of whom insist on standing for their ideals even when they are unlikely to win.
Second, "lesser evilism" disempowers liberal and progressive forces because it gives the Democratic Party no incentive to respond to progressive ideals. Secure in the certainty that liberals will always respond to the demand of "lesser evilism," Democrats can focus their full attention on repositioning their party to accommodate those who might otherwise vote Republican, thus dramatically decreasing the differences between the two parties. And your vote for a lesser evil gives the corporate media the excuse they seek to ignore progressive views during the next four years--the media will say that your progressive views were shown to have no real constituency because you and others didn't vote for the candidates who articulated those views, but chose to empower people who champion the status quo.
Conversely, much as Richard Nixon hurt me personally (by indicting me and sending me to prison for anti-war organizing), the dynamics of his "greater evil" presidency were significantly constrained by an idealistic social movement--and in that context, Nixon responded by recognizing China and by supporting powerful environmental and worker-safety legislation that were whittled down under the Clinton administration. It is the absence or presence of that very kind of social movement that is decisive--and that "lesser-evilism" destroys. Instead of being so sure that "the other guy" is going to destroy the world, better to have a little humility and vote your conscience rather than your crystal ball, because in so doing you make possible a whole different configuration of political possibilities.
Fourth, "lesser evilism" weakens faith in democracy. If people consistently feel obliged to vote for candidates in whom they do not believe, they end up feeling they are without representation, and hence feel that our government itself is less legitimate. Many stop voting altogether. Others feel dirtied by a process in which they have authorized through their vote the actions of an elected official who, acting in their name, supports policies like the death penalty and acceleration of the worst aspects of globalization, which they actually find morally and environmentally reprehensible.