This is an interesting analysis, and goes into why the Left also hates Romney (it's not because it's because he's a Mormon, but because he's rich).
A lot of Republicans hate Romney because he's Mormon, and they've been taught by their ministers that Mormons are an evil cult. This is absurdly false, but it's a serious factor in Republican politics.
They don't dare admit their Mormon-hatred openly, because the Republican Party needs the Mormon vote the way Democrats need and count on the Jewish vote -- a small and much-maligned religious minority, but one that votes as a bloc and contributes time and money far beyond their numbers.
I'm probably not the best guy to analyze this because the whole "I won't vote for him because he goes to the wrong church/wants to marry the wrong person/etc" thing is a divide by zero error for me. I just don't understand what causes the passion. But Orson Scott Card (a Mormon) writes clearly and logically on this.
All I would add is that all those polls showing that Mitt does better in the general election than Newt is strongly related to the fact that nobody knows who Mitt is (except for political wonks) and everyone knows Newt's negatives. In other words, Mitt's negatives will be much higher once the media is done bashing him in the general election; Newt's probably wouldn't be much different.