He will be very hard to stop. Even with conservatives united behind Reagan in 1980, his campaign was on the ropes going into NH. The party machinery was united against him and Bush was flying high coming out of Iowa.
The polls everywhere showed Bush ahead on the eve of the NH primary. Reagan turned it around at the 11th hour through the sheer force of his personality and stupid mistakes by the Bush campaign (i.e. -- the NH debate and "I paid for this microphone Mr. Green!").
There is no unity among conservatives this time.
I'll give you that contribution, but the real reason was that the media was 'behind' Carter, and the republican vote was for the guy who was most likely to enjoy beating up on the media, which gave us continuous entertainment for the next 8 years. That said, I'll give you that conclusion came only from my sample of one, me. I see 2012 as I saw 1980, I'll be voting for the person who will most enjoy easily beating up on the media. That will be the person who the media most enjoyed beating up for the previous 4 years, Gov. Palin (Reagan in heels). Since I don't see this changing, I don't see my vote changing just because RPAC claims they've found a 'candidate'.