I think the Whig Party of the 1850s is also worth revisiting to understand the current state of the GOP.
The same thing is happening to the Dem's...
The parties have moved to the center as the electorate became more closely divided.
This will create a better chance for two additional parties to vie for the vote, and this will affect the majors.
I have never seen the public more equally divided, so I'm not sure how it will play out, but my guess is that a single third party may rise from the split and disaffected Dem's may move to the republicans. This is really what they want and it is the only way to remain the majority party. It has been underway for some time, but the bulk of the party faithful have not yet gotten the message as nobody has spoken about it. They are just vectoring in silence.
That's how I see it.