The Democrats were in a very similar situation in 1992. GHW Bush was riding outrageously high approval ratings and there was no front runner in the race. Bill Clinton, failed governor of a tiny state went on to win the nomination. He was sorta the Dems' tea party guy, see the DLC.
How'd that work out?
That's an interesting point. But I think there's a pretty big historical issue nobody seems to take into account: what I'll call the Second Place Guy.
Republicans love their Second Place Guys:
Reagan finished second in 1976, he gets the nod in 1980.
Bush Sr. served two terms as Reagan's VP. He gets the nod in 1988.
Dole bided his time in the Senate as the de facto Second Placer while Quayle embarrassed himself through his one term as VP. He got the nod in '96.
Bush Jr. was kind of a sequel to the Bush Sr. presidency, which started to look much rosier in hindsight.
McCain came in second place in 2000 after a bitter battle in 2000 (when he was arguably the legit Second Place Guy), shut his mouth, and waited his turn until it came up in 2008.
So who is the "Second Place Guy" to fill in for 2012? Romney or maybe Huckabee or maybe Palin could all make that claim, which only serves to point out that there are at least three Second Place Guys. And really, they all kind of feel like "Third Place Guys" when you look at the polls.
This "Second Place Guy" tradition is a nice fit to the conservative mindset, and I don't mean that in an insulting way. Running a presidential campaign is hard, and so is governing. So go with the obvious guy who has been waiting his turn, because he knows what to expect. That's a conservative (and history shows pretty effective) strategy.
The problem is that the model is broken this year. There is no heir apparent, only a lot of pretty hairy prospects.
You may be right that they have to swing for the fences with an unknown guy, just like the Dems did in 1992. But you also have to see that this kind of thinking just isn't part of the Republican Party tradition.