We don't really know the answer to this because we're in uncharted territory in recent years.
The previous Republican base was built on a triumvirate of three primary blocks of voters - social (evangelicals), fiscal (tax cutters), and military (strong Reaganite foreign policy).
Trump has largely destroyed that construct and created his own cult of personality driven base that is built atop cultural grievance and anything else Trump wants to promote.
When Trump lost in 2020 and attempted his insurrection, there was some degree of blowback in Republican circles, which is how the likes of DeSantis quickly gained momentum before Trump regrouped to take him out.
What will happen if Trump loses this time ? There's a good chance the GOP will somewhat revert to the mean because in the absence of Trumpism and all of the clowns like MJT, Gaetz, Boebert et al., who rode in on his grift, there still is a fairly strong foundation of Reaganite Republicanism that continues to percolate beneath it all. Therefore if Trump leaves, that faction would probably reassert itself through the likes of DeSantis, Haley, Yongkin, Rubio, and a few others.