By your logic, I should not support Santorum. I, as a Puritanical Calvinist, think Roman Catholicism is a heretical belief system that does not preach the true Gospel. However, I do support Santorum in spite of his conservative Roman Catholicism. Why? Because he shares at least somewhat similar political goals. If a Mormon ran that shared my political goals, I would vote for him. I suppose, if all other things were equal, I would vote for a fellow Evangelical Protestant over others simply because they were a fellow Evangelical Protestant. That is only with everything else being equal, however.
Except that there is no Evangelical running, so logically they take the most expressly religious option that most closely matches their own. Does noone remember 2008 and Mike Huckabee? Certain individuals obviously will vote along their policy beliefs, however, as a large voting block, evangelicals tend to vote for the religious choice. To then come out and complain that Mormons vote for a Mormon is very ironic in my objective opinion, given that I am not Mormon, nor Protestant, nor Catholic nor supporting Romney nor Santorum. I find the whole thing very humorous, like a bunch of kettles in a circle.