I agree with Scot McKnight:
Let's agree with a general observation: evangelicals churches tend toward Republican politics while the mainline churches tend toward Democrat politics. I see three problems with this widespread reality in the American church, and I’m sure you can add to this brief discussion of other problems:
First, by politicizing a local church that same local church dramatically decreases its chance of gospeling, teaching, and fellowshipping with those of the other political persuasion.
Second, by politicizing into separate and separable local churches we impede unity in Christ and fellowship with one another, and the longer we impede unity and fellowship the more we built separate cultures that, over time, will be more difficult to find the unity we genuinely have in Christ.
Third, by politicizing a local church the gospel that transcends politics and nation and ethnicity becomes a reduced gospel that forms into a culture that blocks the fullness of the gospel itself.