The Mormon Factor
Chris Cillizza runs down some polling indicating a healthy, albeit surmountable, level of skepticism about the wisdom of voting for a Mormon among Republican primary voters. In some ways, I think this may be a bigger problem for Romney than Cillizza quite sees. The trouble, as I see it, has to do with Romney's convenient conversion to social conservatism over the past two years or so. One assumes that to win, Romney is going to need to talk about his newfound commitment to abortion-banning and gay-hating and the most obvious way to do that would be within the context of talking about his deep Christian faith and so forth. But while that might work great for a Protestant or a Catholic, I don't think it goes over so well if your deep faith is something most Christians consider weird and, indeed, not really Christian.
Similarly, it's hard to do the standard JFK-style "my faith is not an issue" thing if you're simultaneously trying to convince politically mobilized Christian traditionalists that you're the candidate for them. It seems to me that this winds up being a very difficult sweet spot to locate. Indeed, under normal circumstances it would seem almost crippling to me. Romney's good fortune, however, is that the leading contenders are Rudy Giuliani and John McCain, neither of whom are exactly what you'd call social conservative heros either.