If we take seriously the concept that faith affects one's “real life,” then it seems that part of a preacher's job is to dicuss issues (some of which may be political) with the congregation. But I also think a great deal of diplomacy should be exercised, so that people do not feel coerced in one particular direction.
“He has shown you, O mortal, what is good. And what does the LORD require of you? To act justly and to love mercy and to walk humbly with your God” (Micah 6:8). Sometimes acting justly and loving mercy might require political involvement. And perhaps a preacher might encourage people to think about how their poltical choices fit into this paradigm without endorsing a specific means by which he or she thinks this should happen.