He is, after all, running for the party that, has a traditional Christian core. If you believe in a harsh God who gates Paradise, then you care about theological rigor. Mormonism is way beyond the theological tolerance range of conservative Protestantism -- or even less conservative Catholicism. (No-one has dared ask what Pope whatshisname thinks of Mormonism.) From their perspective a vote for Romney is vote for Hell -- only an atheist or Muslim could be worse.
This seems to be hard for many commentators to understand. They assume Romney's vulnerability is based in traditional bigotry. That may be so, but most pundits really don't spend enough time studying theology. I'm as agnostic as they get (functional atheist, philosophical agnostic), but I like studying religion. For religious conservatives, details matter.
Consider the Trinity. Compared to thousand year battles over the relationship of God to the Holy Spirit Mormonism is a whole 'nother kettle of fish.
A sympathetic NYT Magazine article provides us with the an informed rationalist perspective that still sort of misses the point:
Mitt Romney - Mormonism - Mormons - Presidential Election of 2008 - Politics - Elections - New York TimesWell, yes, to a secular humanist all of these revelations are equally respectable -- but to a fundamentalist believer there's a rather huge difference.
.... Still, even among those who respect Mormons personally, it is still common to hear Mormonism’s tenets dismissed as ridiculous. This attitude is logically indefensible insofar as Mormonism is being compared with other world religions. There is nothing inherently less plausible about God’s revealing himself to an upstate New York farmer in the early years of the Republic than to the pharaoh’s changeling grandson in ancient Egypt. But what is driving the tendency to discount Joseph Smith’s revelations is not that they seem less reasonable than those of Moses; it is that the book containing them is so new. When it comes to prophecy, antiquity breeds authenticity. Events in the distant past, we tend to think, occurred in sacred, mythic time. Not so revelations received during the presidencies of James Monroe or Andrew Jackson...
As in an eternity of Hellfire.
That's rather a meaningful distinction!