I'm aware that anti-Mormon bias was expressed by Conan Doyle when he wrote "A Study in Scarlet," and this summer I did read The Mountain Meadows Massacre book, which details an incident that would have justified anti-Mormon sentiment a century ago, but why it persists nowadays I find a bit baffling. I spent a decade in Kentucky, where snake-handlers were considered to be devout Christians - so why not Mormons?
James Fallows addressed the question in October:
The great majority of messages came from people who said: You're wrong, Mormons are an outright menace, and that's because they are Mormon. What they believe is either (a) dangerous in its essence, or (b) an indication of deeper intellectual or character flaws in the candidate, because no sane person could believe the elements of the Mormon faith. Or (c) both...The most humorous comments I've heard were on the December 2 "Mel and Floyd" program broadcast on WORT radio. To paraphrase their banter, evangelical Christians hate Romney because he believes that when you die, you go to another planet and become a god. But this is actually more moderate than that of some other Republican candidates... and perhaps more plausible, because at least we know there ARE other planets.
The only thing I'll add for now is that while I am no fan of Mitt Romney's, I don't envy him having to deal with this sort of thing -- which of course is stronger in the Republican primary electorate than in the population as a whole. Most white people have an idea of what anti-black (or Latino or Asian) prejudice sounds like. Most goyim have an idea of what an anti-Semitic rant sounds like. But even having grown up in an area with lots of Mormons and had LDS friends all my life, I hadn't been fully exposed to the intensity of anti-Mormon views [example at the link]...
They asked the question: If during a debate, Romney shakes hands with an anti-Romney, might that result in a huge explosion?