Daniel Larison explains the inherent difficulty:
The impossible balancing act is stressing the political irrelevance of the theological differences Mormonism really does have with Christianity while simultaneously claiming that this very same religion, whose distinctive substance is supposed to be irrelevant, informs and shapes his “values” that he will rely on to make judgements about policy. Another part of the balancing act (which is where it becomes really dangerous politically) is to declare that it is “un-American” to judge a candidate based on his religion without insulting the millions of voters who consider a candidate’s religion an important part of selecting their preferred candidate, while also paying homage to the “separation of church and state” without actually endorsing the idea that the separation of church and state has any constitutional basis (which a fairly large number of religious conservatives doesn’t accept). His speech will have to go something like this: “My faith, which is very important to me and has made me who I am, should not be important to you, but it is important that we have a person of faith leading this country, and that person happens to be me.”
In the end, Romney has always been in an impossible position: a sizeable percentage of his own party will never vote for someone of his religion, and these are the same people he needed to win over to become the unchallenged social conservative consensus candidate, which is why Romney’s campaign has always been a fool’s errand as I’ve said from the beginning. My guess is that Romney gives the speech on Thursday and his campaign in Iowa begins to implode, as his shallow support there evaporates.