Atrios gets this
almost right, especially the part about the media not ever going too deeply into what a particular candidate's faith actually is. But that is only part of the dynamic of Romney's religious affiliation, and it's not the most important part.
Evangelical Christians are now known for tolerating political alliances with people who don't really share their beliefs. Having a largely irreligious Reagan and then Bush I as president wasn't much of a problem because the Religious Right was still in the process of coalescing into a coherent movement and because Reagan and then Bush managed to say the right things.
George W. Bush, though, has managed to enjoy such a powerful cult of personality because he didn't just say the right political
things, but the right religious
things. Many Evangelicals believe that Bush is an Evangelical Christian just like them. He believes the same things, has the same worldview. I can at least say anecdotally that some Evangelicals who are unhappy with Bush's job performance have still not wavered in their views of his status as an Evangelical believer, and therefore are able to still consider the GOP as "their" party.
The point of this is to say that it's possible that George W. Bush has spoiled them. They elected one of their own, and then they were able to reelect him. He has pursued their agenda, they believe, blaming any lack of progress on the Democrats first and insufficiently Christian Republicans second. When the next election comes around, they want another person who believes just like them.
That's the root of McCain taking his religious pandering to such extremes this time around. He understands that if he wasn't sufficiently pious in 2000, there's no way that he can be so in 2008 without major changes in his message.
Mitt Romney is the odd duck, because he is a strong believer in a conservative religious group that is very similar to Evangelical Christianity. However, Evangelical Christians are the people in this country most
likely to not only see the differences that exist but to consider them dealbreakers when it comes to nominating a candidate for the presidency. Further, quite aside from the actual differences, there are stories told about Mormons in every Evangelical Christian congregation - every one. Mormons are mocked, attacked and feared. If a pastor decides upon a "cult series" for Sunday nights, Mormons are listed right along the Moonies, Heaven's Gate and Jim Jones.
That Romney's past statements about abortion and homosexual marriage are coming to light will only strengthen the case against them voting for a Mormon; obviously besides all their kooky religious ideas, they simply can't be trusted as true conservatives.