I don't think preachers (who are over a church) should endorse certain candidates or issues that are to be voted upon from the pulpit. I have heard a preacher on the radio asking people to vote on a certain proposition, but the preacher on the radio was no longer the senior pastor of the church as he'd passed the reins on to younger leadership, so he could give his endorsement and not worry about the church losing its tax exempt status. Now, what they do at my church is they say something like "Hey everyone, it's almost time to vote. If you are not registered to vote, come on over to the community life center after service and get registered. While there, you'll find literature on the different candidates and issues and proposals. We can't tell you what to vote for or who to vote for, but we want you to be well educated and participate in this election, because this year is proving to be an important election year. Don't let your voice go unheard."
Since there's supposed to be a separation of church and state, I think that pastors shouldn't endorse candidates or issues from the pulpit. That's why Sarah Palin left one of her past churches, after she asked the congregation, while she was a politician, to pray that attacking Iraq was God's Will. I'm sure the leadership of that church didn't like the idea of losing their tax exempt status.
However, that doesn't mean they shouldn't allow candidates to come visit the church, but, they SHOULD encourage the congregation to get educated on the candidates and issues, register to vote, and make informed voting choices.