After a two-day detour to Rush-ville, we're back to talking about Romney's lack of biographical explanation for his 20 year rightward trend. Apart from naked political ambition, there is no other justification given for his conservativism, and I think that's most of the reason that the Right hasn't embraced him. With Rick Santorum, you understand the source of many of his views. For instance, he caught a lot of flak for coming out against prenatal testing, on the grounds that it would encourage abortions if a disease such as Down Syndrome is discovered. Given that Santorum has a daughter with Trisomy 18 (extra copy of chromosome 18, where Down Syndrome is an extra copy of chromosome 21), his views on the issue make sense.
If Romney had a similar story, conservatives could trust him more. As it is, we have statements from all over the map over the past 20 years. Even recently, he has occasionally made statements that call his conservativism into doubt, such as his initial statement opposing the Blunt amendment or his recognition that cutting spending would at least temporarily slow the economy. If he's elected President, how could anyone, conservative or liberal, claim to know what Romney will do when it really matters. He might return to his more moderate/liberal roots when the fate of the world is at stake.
For a movement demanding ideological purity, like the Tea Party, you have to have either a long history of demonstrated adherence to dogma, or a biographical explanation to justify your lack of such a record. Romney has neither, and because of it he is incapable of being truly accepted by the GOP.