The Republican Party is no longer a mainstream political institution. It is a radical, anti-American, anti-Constitutional rightwing insurgency -- our own jihadis. It is dedicated to the overthrow of our government.
I think America is still solid enough to defeat this extremist effort -- that we're not yet in real danger of succumbing to it. Therefore, I think the most likely outcome is that the GOP winds up withering away, a la the Whigs, and some new right-of-center party emerges to express the libertarian perspective. Or maybe it just fragments into its various components -- evangelical, business, racist, libertarian, etc.
But they're not even hiding or euphemizing their agenda any longer. They're fed up with America, and are not gonna take it anymore.