Here's a great blog about a NY Times article on an amazing preacher. He refused to use his position as a religious leader to endorse politicians or political ideologies. Of course historically religion and politics have gone hand-in-hand. The idea of separating church and state, while not new (see The Greeks) was one of the most revolutionary concepts of The Enlightenment. After all, government that claimed its authority from a god could not be questioned. Thus a separation of church and state was needed so that government would have to derive its authority from the people -- and thus be responsible to the people.
Still, in many governments today we still see religious leaders with tremendous political power. Most people would say "oh that's only the case in backwards, third world countries." Enter Exhibit A: George W. Bush.
Bush was elected (though not without great controversy) but has repeatedly invoked the name of god as an integral part of his administration. He's made statements that he thinks he's on a mission from god. He's used his personal religion to discourage scientific research. He's tried to influence the education system to promote religion over science. His tactic of claiming "treason" when somebody disagrees with his wars is not unique to him or to religious leaders. Still it's no different than claiming "heretic" when confronted with criticism.
There's a huge difference between Bush and the ayatollahs in Iran. Despite his fondness and pride in his religion, Bush is not a religious leader. He has not spent his life devoted to a religious following. No, he is a politician. He simply uses religion. These churches with their American flags and "no gay marriage" banners are just puppets of the GOP.