Certainly not all Evangelicals have this attitude toward Mormons. But Bill Keller is a fairly prominent evangelist - at least he's well known - and this screed is the kind of thing that Romney can expect from some on the religious right:
Keller, who in 2007 was in the national news after coining the phrase, 'A vote for Romney is a vote for Satan," said, "It is sad to see a man of God like Franklin Graham go into the nation media and state that he would vote for a cult member like Romney whose beliefs lead the eternal souls of men to hell. Osteen is a gutless coward, nothing more than a motivational speaker, who ignorantly lies to people by saying that a Mormon is a Christian, when Mormon doctrine is 100% inconsistent with Biblical Christianity. Starr, well known for his Christian views, made the political case why he could vote for a Mormon without once dealing with the spiritual implications of having a cult member as President."
Keller went on to say, "People are free to believe whatever they want. I have no problem with a person of any faith running for office. The only thing I have ever asked Romney or any member of his cult like Glenn Beck to do is to tell the truth. Romney LIES when he calls himself a Christian, since he and members of his cult believe their 'jesus' was created through the sexual union of their 'god' who was once a man, and Mary, meaning their 'jesus' is NOT a deity, is the brother of Lucifer, and once visited the United States. The 'god' and 'jesus of the Mormon cult is NOT the God and Jesus of the Bible, and the final authority for Mormons is NOT the Bible, but the writings of their cult's founder Joseph Smith who was a documented con man, pedophile, polygamist, racist, and murderer."
No one has any idea how common this attitude toward Mormons is on the right or how it will play out in the primary process and, if it works out that way, the general election. It will be partly offset by the enthusiasm of Mormons who make up a sizable voting bloc in some western states. But it will hurt Romney's chances in several southern primary states and could affect a close general election contest.
Most mainstream protestant religions recognize the LDS as a Christian sect. That doesn't seem to matter to Keller who is wont to see Satan in a lot of places, and not just in the Mormon religion.