On the off chance that a person is able to get liberals to admit that the current seemingly unstoppable rightward drift of American politics is a problem, and that the current Democratic Party represents a corporate-owned center-right party of (slightly different) war and (more creative) austerity, that person would then be faced with the inevitable appeal to the "lesser evil;" sure, Democrats are intolerably weak, but Republicans must be stopped or else they will force-impregnate every woman in America and launch our entire nuclear arsenal at imaginary Iranians on the moon. As a commenter at Lawyers, Guns and Money solemnly put it, "every time you don't vote for the lesser evil, people die."
And, I have to admit, as a short term fix this argument carries a lot of weight. No one wants to see President Santorum nuke the moon. As a long-term strategy however, it's a disaster, and we've been hearing it since at least 2000 and probably long before. The 2000 election, of course, is seen as the defining argument in favor of lesser-evilism, and liberals love to place the Bush years on the conscience of those privileged hippies who tried to make an idealistic political statement by voting for Ralph Nader. (I'll have more to say about why supporting third party candidates is an ineffective way of pressuring Democrats in a future post.)
The problem with lesser-evilism, though, is that, while you may get less evil now, you will surely get more evil later. Because your tepid, center-right New Democrats won't win every election. This is still a balanced, two-party system, and the public's preference inevitably swings back and forth from one party to the other. Talk of a permanent majority, of shutting the other party completely out of power for a generation, doesn't work. It was a fantasy when Republicans were kicking around the idea in 2004; and it was equally so in 2008 when some Democrats were crowing about the complete obliteration of the old Republican Party, and how that party would have to reinvent itself to survive. It did reinvent itself, of course, by driving even harder to the right and coming back to win the midterm elections in decisive fashion.
So while liberals mock the "heightening the contradictions" crypto-Leninist model of third-party voting (or abstention), their own embrace of essential lesser-evilism is functionally not any different. It just kicks the can down the road a few years. The long march to the right continues unabated, and they have no answers for it. This year, we have Rick Santorum and Newt Gingrich, two fringe nuts who would have been laughed off the stage as fringe kooks ten years ago, being serious presidential contenders who have successfully forced the "moderate" Mitt Romney to move rightward to match them. And while Romney likely won't win the 2012 election, one day another Republican will ascend to the presidency who, enabled by a wobbling Democratic Party and its steadfast supporters of lesser-evil voting, will be even more extreme than George W. Bush, just as Bush was more extreme than Reagan, and Reagan than Nixon, and Nixon than Eisenhower, and so on.
I'd be negligent not to note the final irony, which is that liberals like to pat themselves on the back for being "realists" who understand the political system, while scolding naive "emoprog" puritan idealists, when their own blind spot is a fantastical belief in a permanent Democratic coalition of sensible technocrats, which should have already been blown to bits a couple of years ago.