Imagine an alternate universe in which racial integration never took place in civil society (such as in baseball) or in law. Slavery was abolished mainly for economic reasons. The 13th, 14th, and 15th Amendments didn't exist. But where, otherwise, the world is pretty much the same, and other social revolutions did take place: the Sexual Revolution, feminism, gay rights, theological liberalism, etc. In this universe, white supremacists win both the Democratic and Republican nominations for President.
The Democrat is a eugenicist, who believes whites are the most evolved of the species and wants "inferior" races to die out in America through population control and forced sterilizations. The Republican, however, believes that slavery of "people of color" is most ideal, and failing that insists on the the segregation of free minorities.
Aside from this racism, the Culture War rages on. The Democrats are the party of abortion, sex eduction, evolution. The Republicans are the party of aboriton restrictions, abstinence-only education, and creationism.
In foreign affairs, Repubicans insist on "going it alone" so that White America polices the world essentially by itself, whereas Democrats are multi-lateral, believing White America should rule the world with the help of, and in cooperation with, Northern Europe.
Domestically, Republicans and Democrats go back-and-forth, issue-by-issue, whether federal control, or state and local government, would best preserve white supremacy.
If you lived in this universe, who would you vote for?
The godless Democrats? Or the Republican theocrats?
Or will you say, "Wait! Those distinctions are hollow at best! They're both evil!"
Okay, so they're both evil. But don't you have a moral responsibility to choose the lesser of two evils? All that democracy asks of you is your best judgment. You won't be held personally responsible for the consequences.
"Well, I'll vote for a third party instead! Especially for one who shares my anti-racist values!"
The typical response to that is, "Go ahead! Throw your vote away!"
If you found yourself in this alternative universe, and are a marginally decent person, you would not vote for either the Democrat or the Republican. Not voting at all is more legitimate than voting for either one. Your vote implies your consent or approval, and if you are a decent person, you would withhold it, and all arguments that you should vote for the "lesser of two evils" would sound ridiculous to you. After all, your vote is far more likely to be uncounted or miscounted through error or fraud than to be the deciding vote.
Because racism is particularly revolting, many people would be persuaded by the argument that it is wrong to vote for either party. But in our universe, both parties are explicitly anti-racist, and so many would think the anti-Two Party argument doesn't apply to us.
But the Democrats and Republicans do share values that are nearly as repugnant as racism would be. I'll mention just three.
First, that the President should have power to commit acts of war against countries that do not threaten or attack us. And they call this "humanitarian intervention." But the U.S. would be the first to object if some other country did the same thing. If France and Russia announced that Castro is a "brutal dictator" and that they would invade Cuba, overthrow him, and install a democracy, wouldn't Americans be the first to protest, calling them hypocritical and wondering why they won't just mind their own business? Although we're no friends of Castro ourselves, wouldn't we try to stand in the way of this supposed "crusade" for liberty and democracy? So why do we wonder why foreign poeples think the same of us?
Second, that the Bill of Rights should be ignored whenever Executive Power is inconvenient. One example of this is civil asset forfeiture. Another is warrantless spying on American citizens. Far from protecting us from terrorists, it appears the main reason for the government to have this power is to spy on and blackmail critics and political opponents; as Paul Craig Roberts once said, this should be obvious even to a naif.
Third, and most pertinent these days, they have the same philosophy of wealth creation. Whereas real wealth is created by human ingenuity and cooperation, the Two Parties seem to agree that access to credit is what creates wealth. And so they set up a system by which banks may lend out ten times what they have in deposits, where a central bank is the "lender of last resort" and the dollar is not backed by gold or silver but by . . . nothing. The result is not steady growth with better products at lower prices over time, but rather a cycle of booms and busts and rampant inflation. When things get really bad, as they are today, the solution they always resort to is . . . more credit. But pumping fake money into the system doesn't stimulate growth, it only creates price distortions, leading people to make poor economic decisions, leading to yet more mal-investment.
If the Democrats and Republicans were in agreement on just one of these issues, that would be enough reason to refuse to vote for either. Yet they agree on all three and more, and their "bitter disagreements" are based more on execution than principle.
So even if I may guess that either Obama or McCain may be marginally less bad than the other, that is no reason to express my approval of one or the other, especially because there's no chance my vote will determine the winner. It is far more sensible to vote for a third-party candidate more in line with my views, or to even play the lottery instead.