It is indeed impossible to keep religion out of politics, if by religion one means morals, values, ideology, or worldview. Even saying that there should be no religion in politics requires some sort of religious position.
Of course, if all someone means by "religion" is a particular religion's ceremonial practices, then of course we can keep that out of politics.
But even prayer - seeking the counsel of God - why should an elected official be kept from doing that in public any more than the official seeking the opinion or advice of anyone else?
http://walrus.blogtownhall.com/2007/09/06/is_jesus_a_republ ican.thtml