There is a difference between a moral issue and a political issue.
This affects how I preach (at least, how I intend to preach).
A moral issue is about right and wrong, about what we ought to do. This is about ethical systems, which can be talked about independently of political systems.
In preaching the biblical texts we are confronted by many moral issues. In the New Testament the focus is more on morality than politics. Jesus refuses to be an earthly political leader. Jesus says his kingdom is not of this world. But his kingdom ethics will impact all human institutions, including politics. (Whereas the opposite is false; viz., human institutions do not impact the ethics of the kingdom of God.)
In his famous book The Politics of Jesus John Howard Yoder refers to Jesus as "apolitical." Jesus refuses to align with any of the preexisting political parties, and critiques them all.
But Jesus does talk about right and wrong, about what a person ought to do and what a person should not do. Jesus talks about "righteousness" and "unrighteousness." He demonstrates how it is possible to morally discourse while not being political.
For example: It's wrong to rape children for fun. A preacher can preach on that without touching politics. My intent is to preach on moral issues, as they arise in biblical texts, without touching politics.
Moral issues can, and should, affect political systems. If all political leaders in the first place adhered to the ethics of Jesus our world would be different. (On the ethics of Jesus see, e.g., Richard Hays, The Moral Vision of the New Testament: An Introduction to New Testament Ethics. For a practical example of how the ethics of Jesus can affect political systems see Ronald Sider, Rich Christians In an Age of Hunger: Moving from Affluence to Generosity.)
Some people conflate morality and politics and view preaching on ethical issues as being political. This is a false equation. We have to say things like raping people is wrong without being accused of preaching politics.
Preach issues, not politics. Preach the ethics of Jesus, and let all political parties be measured by this.
(Greg Boyd recommended that I read Christian Anarchism: A Political Commentary on the Gospel.
On the relationship between Christianity and politics see my post "Jesus-Following and Politics.")