Andrew Sullivan sums it up nicely:
The problem with Christianity for those who seek earthly power is that Jesus explicitly renounced such power. Socialism and left-liberalism and "compassionate conservative" are really devices with which the state assumes the moral obligations of the individual - and increasingly robs the individual of the resources to be charitable herself. Christianism - of both left and right - is not just a variation of Christianity. It's an attempt to coopt Christianity to empower the political leader. That's why the politicians like it: it gives them the moral highground, and more money, and eventually more power. All of which leads to less freedom and less genuine faith.
Every moral crusade begins with the purest of motives and with a genuine desire to improve everyone's quality of life. Unfortunately the cost of bending the full force of governmental power to achieve those ends comes with a very high price tag - one nobody would willingly pay if the cost were made apparent up front. Coerced virtue is no virtue at all, and the damage that coercion does to the human spirit affects the one doing the coercing as surely as it does the one being coerced.
Power corrupts. Use sparingly.