Here is a study of Mormon bias and politics out of Vanderbilt Univeristy. The key findings are:

Bias against Mormons is significantly more intense among the public compared to bias against women and blacks. The bias against Mormons is even more pronounced among conservative Evangelicals. Their bias against Mormons rivals their bias against atheists.

Only about half the nation claims to even know a Mormon or to know that Romney is Mormon.
The extent of the bias against Romney is moderated if the individual already knows that he is Mormon. That information seems to demystify the Mormon religion, making people more tolerant of the religion. Those who do not know Romney is Mormon exhibit much greater bias upon learning of his religion.

When participants in the survey are provided information that stereotypes Mormons, such as ‘Mormons are part of a non-Christian cult” or “Mormons are polygamists,” they react negatively to Romney’s candidacy.

Participants react favorably to messages that dispel the negative stereotypes about Mormons. Examples would be “about a hundred years ago the Mormon Church banned polygamy,” or “the Church of Jesus Chris of Latter-day Saints stresses traditional family values.” However, simple appeals for religious tolerance do not win over support for Romney from the respondents.