In his book The Case for Civility, Os Guinness describes two ways the independence of our faith is strategically compromised when churches try to advance their interest politically. He also describes why this must fail to address the deterioration of our culture, and I think his argument is compelling. This is from p. 101.
Faith’s loss of independence through politicization is more damaging than it might appear, for the cultural captivity of the Christian Right represents a double loss of independence. Rather obviously, Christians lose their independence when they engage in politics in a way that allows their faith to become subservient to politics and its priorities and procedures. But less obviously and equally important, Christians have already lost their independence when they attempt to find political solutions for problems that are essentially cultural and prepolitical — in other words, when they ask politics to do what politics cannot do.
When there has been a profound sea change in culture, as the United States has experienced since the 1960s, it is both foolish and futile to think that it can be reversed and restored by politics alone. That approach will always fail, and can only fail. Politics is downstream from the deep and important changes in American culture, and what lies upstream is mostly beyond the reach of political action. Thus overreaching political activism is bound not only to fail, but to leave the cultural changes more deeply entrenched than ever and those fighting them weaker than ever.
So instead of using political methods, like mobilizing church members to support or oppose certain political candidates or ballot measures, churches should simply teach the Word as it applies to the moral, ethical, or social questions implicit in the political debate. Then the members can act individually, based upon their informed conciences. That action would certainly not be limited to voting. The more powerful actions would be things like speaking the truth in love to those with whom our lives intertwine, and reflecting the mercy of Christ in our deeds.
Sometimes individual Christians (even ministers) may have opportunity to speak out publicly, but we should distinguish between speaking as individuals and speaking as the Church. When conducting a service, teaching a Bible class or counseling, I speak for Christ at the behest of His Church. When writing a blog post or speaking in a hearing before the town council, I voluntarily speak only for myself, a member of a particular community. Every member of a church has a similar private voice, which can collectively have a powerful influence upon our culture. However, this voice is not a tool to be manipulated directly by churches, because that would turn a prepolitical influence into a political one, simultaneously weakening it and compromising the independence of our faith from the political winds.