The voters have already rejected the GOP. The number of GOP voters is already highly exaggerated this year because there is a real candidate - a choice - available. If Trump loses, the GOP loses, likely permanently, well in excess of 10 million votes in the aftermath. These are votes that may well simply disappear (they won’t go to Democrats, merely disappear from the system due to non-participation)never to reappear again in our lifetime.
It will not be at all difficult to give up on the American political system and simply disappear into the political underground - and millions will take that route as the system has nothing for them, nothing at all.
I guarantee there will be a new party as a result, and the GOP will fade into obscurity.
Did anybody really vote for Romney because they liked him? Or did they just vote for him because he wasn’t Obama?