Why? It's not because politics and religion don't mix - they do. Christians ought to speak out about the big social issues of the day, and stand up to politicians when policy is wrong. The Church of England has a long and glorious tradition of irritating the heck out of Government by making the point that God does have a lot to say about politics. This, by and large, is a healthy thing (as long as we get our facts right, otherwise it's a bit embarrassing). Nor is my problem with Christians in general getting involved in politics. Party politics is an integral part of the democratic system, and anyone who wants to get in there and make a difference needs to join a party.
What makes me twitchy is when I hear people who are known publicly as Christian leaders - clergy, pastors, etc - promoting their political loyalties. And that's the nub of this: of course ministers should have political opinions, but should they have political allegiances? I would really prefer them not to, for two reasons:
One - it creates a clash of loyalties. It's very difficult to take that courageous stance against the government if you happen to be a member of their party. You could end up in a position where you have a prophetic critical witness one day, and the next day you go very quiet because your lot have got in. This is very seedy.
The second reason is what it does to your witness. The church is called to reach out to everybody, without exception (see previous post, and loads of posts before that). As ministers, we are the public face of that mission. I don't really see how I can really reach out to everyone if I'm simultaneously saying "vote David" or "vote Lib Dem", or whoever else. Doesn't that send out the signal that the gospel is only for the people who vote the way I do? I think it does. Just saying.