Pundits and analysts say the 35 or 37 percent Trump approval crowd is a "Republican base" but its history suggests otherwise. While people in said group are conservative, their voting history is more skittish. Ross Perot, Ron Paul, Dixiecrats, and other Independent candidates have won their votes instead of GOP candidates --and still others have won their votes in state level races.
As far as being reliably conservative, they are; as far as being reliably Republican, not so much.
So a 35% vote becomes more like 17% when you look at reliability of the conservative base vote as GOP fans.
Such phenomena best explains "moderate Republicans"--and why the GOP is never going to give this "base" 100% of what it wants. Even when GOP has been reliably conservative itself, it has found it self left in the lurch sometimes.