Although there are Christians who--for reasons passing understanding--align themselves with the Left, there can be little doubt that the Left as a whole is, at its core, a fundamentally anti-Christian movement. Generally speaking:
The Left denies the core, fundamental doctrines of the Christian faith. Among the core teachings of Christianity are that in Jesus of Nazareth, God became human, lived a sinless life, died an atoning sacrifice for sin and bodily rose again. While there are a few on the Left who may still believe these doctrines—just as there are many on the political right who deny them—the vast majority of those who deny these doctrines are on the Left.
The Left often seems willing to support any religion other than Christianity which they ridicule and vilify. To many on the Left, when someone wanted to use public tax dollars to paint a picture of a Christ-figure dipped in urine, that was Constitutional free speech at work. If someone did the same with an image of Muhammad, that would be hate speech worthy of prosecution.
If a Christian teacher wants to keep her Bible on her desk, the Left protests that this is a violation of the separation of church and state; but if a public college wants to spend tax money for Muslim foot washing facilities, the Left is mute.
The Left successfully works to strike down the placing of memorial crosses on public land (or the Ten Commandments in courthouses, or Nativity scenes in town squares, etc), but is silent as the Leftist Obama administration pays $16,000 in tax dollars to send a Muslim Imam to Saudi Arabia to promote American-Muslim and Saudi Arabian Muslim relations!
The Left denies the biblical teaching that people are sinners in need of repentance (Jeremiah 17:9; Romans 1:1-3:21) and believes instead that people are basically good. To those on the Left, the problem is not that people are sinners, the problem is a bad environment or bad parenting, or poverty or even religion--anything but sin! In fact, since people are basically good, bad behavior must always be the fault of someone or something else (which has created a whole "industry" of victimization).
This belief in the fundamental goodness of people is at the heart of many of the Left’s social programs. The Left reasons, if we could just educate people, they would be good (one need only think of highly educated criminals to see how this philosophy has worked out).
If we could just give them an income and good housing, they would be good (one need only think of the word “projects” to see how that philosophy has worked out).
In foreign policy, the problem is never that our enemy is bad and the problem is never the enemy’s flawed ideology or religion. The problem probably lies with us so if we just tried to understand our enemies and negotiate with them we could always come to an understanding, since all people are basically good (So after 10 years of negotiating with Saddam Hussein, many on the Left still wanted to continue negotiating while Saddam Hussein continued torturing, starving, murdering and robbing his own people).
The Left supports extreme cultural relativism. Churches that do not allow women to be priests or pastors are denounced by the Left but Muslims--none of whom allow women to be Imam's--are excused.
In fact, the Left will not even condemn those Muslim leaders that refuse allow women to drive or go to school. Nor will they condemn individual Muslims who beat their wives or commit honor killings (who are we to judge their culture, says the Left).
Similarly, the Left endlessly condemns America for imperialism (in taking Indian lands) and for slavery, but they refuse to condemn Muslim nations whose history of imperialism is among worst in human history and who enslaved far more people than America ever did.
The Left supports extreme moral relativism. Almost any sexual perversion is excused on the basis of, “who are you to judge?” So while global warming and harming the environment are modern sins created by the Left, biblical sins like sex with people of the same sex, sex with multiple partners, violent sex, and even sex with animals are behaviors seen as alternative (and valid) lifestyles.
The Left supports the murder of unborn children. Forget the religious/philosophical arguments about when a fetus becomes a person. Biologically a woman’s fetus is a living human being. Abortion is the killing of an innocent human being. Some on the Left (e.g. Barack Obama) are so vile that they even support the partial delivery of this little human being while they scramble its brains! Some even want to force you to support this abomination with your tax dollars!
Of course not all on the Left agree with all of these positions, but is it beyond reasonable debate that these positions characterize those on the Left. They do not characterize those on the right.
Sometimes Christians have to hold their nose, so to speak, to support a candidate on the right. Sometimes the only available options are so bad, we may just refuse to support either side. But it in most cases, it is really beyond my understanding how born again, Evangelical Christians can support the Left in good conscience.