Why is it off limits to say that Romney is a Mormon??

[Spike89]

14. It's subtle...we can discuss Romney's faith

I think there is a very blurry line where it is appropriate to examine a candidate's faith because after all it does (the candidate's themselves claim) inform how they make decisions. The blurry part is "a candidate's faith" which can be parsed in two different ways; his/her personal interpretation and implementation of the religion or the religion itself.
It is fair to look at the first and discuss, for example, "Romney's faith has lead him to take a stand on issue X, and therefore he's likely to..." It isn't fair to say "Mormonism is a cult and magic underwear is silly so Romney, as a Mormon, is unqualified to be President."
Simply put, it is one thing to attack a candidate for "being" a member of a religion (wrong) and another thing to attack that candidate for "acting" on the religion. Not every Catholic is anti-choice, not even every anti-choice Catholic would impose that view on their legislative agenda. Likewise, not every Mormon will govern as an officer of the church.
Obama obviously isn't muslim, but it doesn't matter to me. He once (still does?) belonged to Rev. Wright's congregation, but again, I don't care. What matters is how those associations inform and influence his behavior and policies that matter.
I'd rather have a member of some crazed cult who can legislate sanely than a member of the most "sane" established religion who legislates on ideology over practicality.