A recent Associated Press article cites "an ABC News-Washington Post poll conducted July 18-21" that showed "that 32 percent of those who said they leaned Republican described themselves as 'uncomfortable' with the idea of a Mormon president.
An earlier poll by the Pew Research Center said 30 percent of respondents said they would be less likely to vote for a candidate that was Mormon. The negative sentiment rose to 46 percent for Muslim candidates and to 63 percent for a candidate who "doesn't believe in God."
Pollster Andrew Kohut, Pew's director, said that between the late 1960s, when Romney's father ran, and now there has been "one of the great transformations of our era. There is more mixing of religion and politics than there was then. As a consequence, people scrutinize Mormonism — or any other religion — more closely than back then."
He cites the growing influence of the Christian right, the political activism of tele-evangelists and a trend that has seen a steady migration of Christian conservatives into the GOP fold, particularly in the South.
It is noteworthy that more Republicans are uncomfortable with a theoretical candidate who would disbelieve in a supreme being, most likely due to lack of evidence, than a very non-theoretical one (Mitt Romney) who apparently professes adherence to notions including the brotherhood of Jesus and Satan and the magical gift by an angel named Moroni of a new and improved version of the bible to Joseph Smith Jr. in upstate New York in 1823.
Pollster Kohut's summary above is apt, including the emphasis on "Christian conservatives." Christians aren't doctrinally Mormon. Thus we have this other article: "Conference aims to counter attacks on LDS doctrine, history," in the Salt Lake Tribune. Attacks from Christian conservatices, mostly. Mitt Romney wants to be president; a lot of his fellow Mormons would like him to be president, too. But that's unlikely unless Mormonism can be seen by the masses of GOP-voting evangelicals as being something acceptable, and not a demonic cult. In the end, Romney may claim strongly enough to dislike abortion providers, Democrats, gay Americans, atheists, and the French that evangelical Christians will organize and vote for him: first to be nominee, then in the general election. Let's see if the conferences and LDS apologetics work. But somehow, I don't foresee the appearance of "Have you hugged a Mormon today?" bumper stickers in mega-church parking lots anytime soon, unless they're printed as a call to evangelization. But even being an evangelist to Mormons certainly doesn't require voting for any.