Quoting from James Rydberg at YouGov’s Model Politics blog (emphasis is mine):
A recent YouGov survey asked 1000 respondents if they would be willing to vote for “a generally well-qualified person” nominated by their party if they learned that candidate had one of the following characteristics. These characteristics range from criminal actions to the candidates’ faith, but consisted primarily of characteristics the public generally views negatively. …
This certainly does not mean that these characteristics determine an election, but this can provide some guidance to candidates: Mitt Romney may not need to worry about his faith, but Tom Cruise might.
You’ll see that Mormons come out mostly neutral, but atheists, Muslims, and scientologists fare worse.
I’d be curious to see, though, whether Republican respondents rated Mormons differently than Democratic respondents.
Update (Nov 24, 2010): Rydberg tells me that there’s not a big partisan difference. In fact, when Republicans and Democrats are asked separately, Republicans are slightly more favorable toward Mormons (but very slightly).