The New York Times points to an early oddity in the 2008 Presidential race on the GOP side: the majority of likely candidates are varying degrees of moderate. They list Giuliani, Pataki (hard to believe, but it looks like it), McCain, Frist, and, of course, Mitt Romney, supposed Governor of Massachusetts.
It's tough enough for a moderate to have shot at winning in Republican primaries given the conservative dominance of the process, but splitting the moderate vote would make it nearly impossible. A moderate would need to be the moderate candidate against a split field of conservatives.
So far some of the candidates such as Frist and Romney are trying to rectify their situations by changing their positions. Luckily for Frist he's had a head start, switching his posture once he became Majority Leader.
For Romney the problem is much harder. He started in elective office only of 2 1/2 years ago, which he won by campaigning as a prochoice social moderate. His conversion seems to have started after the absolute failure of his campaign to add more Republicans to the state legislature in 2004. It culminated last week in his veto of an emergency contraception bill, a veto almost sure to be overridden. His ability to convince the conservatives that his latest "evolution," as he puts it, is legitimate will be the test that determines whether he'll be a credible candidate.