There are, generally speaking, two reasons to give your vote to someone as president. The first is because you are inclined to vote for that person. The second is that you actively wish to deny another person the presidency. In this case, I am voting for Obama because I wish to do both: Vote for him, and vote against Mitt Romney.
… Look: The modern national Republican party is a hot mess, a simmering pot of angry reactionaries driven by selfishness and willful ignorance, whose guiding star is not governance but power, and whose policies and practices are tuned to build an oligarchy, not nurture a democracy. Its economic policies are charitably described as nonsense and its social policies are vicious; for a party which parades its association with Jesus around like a fetish, it is notably lacking in the simple compassion of the Christ. There is so little I find good or useful in the current national GOP, intellectually, philosophically or politically, that I genuinely look on it with despair and wonder when or if the grown-ups are ever going to come back to it. Before anyone leaps up to say that the modern Democratic Party has problems of its own, know that I do not disagree. But if your practical choices for governance of the country are between the marginally competent and the actively malicious, you go with the marginally competent.
… More to the point, I think the real problem is that the actively malicious, awful and small-minded politics of the modern GOP have to be stopped. The modern GOP, simply put, has no moral center; it pays superficial obeisance to “traditional values” while yearning to implement policies whose highest moral achievement is consolidating wealth for the very few, and is perfectly happy to be as cynical as it needs to be to achieve that goal. If the GOP wins this election, it will simply become further untethered from the common good of the nation, because why shouldn’t it? There is no political reason for it to be otherwise. If mendacity continues to be rewarded, then mendacity is a legitimate strategy of power.