I'm a strong believer that a Christian worldview must intersect with all aspects of life--including one's public life. From a Biblical perspective, this understanding starts from the Cultural Mandate (Gen 1:28). Nancy Pearcey explains,

"The first phrase “be fruitful and multiply,” means to develop the social world: build families, churches, schools, cities, governments, laws. The second phrase, “subdue the earth,” means to harness the natural world: plant crops, build bridges, design computers, and compose music. This passage is sometimes called the Cultural Mandate because it tells us that our original purpose was to create cultures, build civilizations—nothing less."

This train of thought is carried out furthest in Reformed theology. Calvin had a view which went so far as to suggest Abraham Kuyper's sphere sovereignty is a great place to start. Contemporaries such as Francis Schaeffer and Nancy Pearcey have written thoroughly on the subject of Christian engagement in culture and public life.

I especially like how Os Guinness puts it in his book, The Call. He says we have a primary calling to be disciples of Christ. Flowing out of primary call, we have four callings to do: in the areas of church, community, family, and vocation. Our involvement in government is intricately tied to this secondary call to community; though government clearly has implications for all of these realms.

Once we understand that our political views should, nay must, be informed by our Christian worldview, the difficulty then is figuring out what that means.

I personally place a high level of emphasis on biblical theology. I find that very few Christians who write on political issues start with any true Biblical exegesis. Contemporary guys like Jim Wallis, Shane Claiborne, and Ron Sider seem to start with their own political assumptions and weave in Biblical verses where it seems to fit.