RINOs aren't protecting Democrats - they are protecting a system that keeps public opinion from removing undesirable incumbents.
If Trump is the nominee his public profile and big mouth can keep the heat on the fraudsters, even if he has no electoral path to victory. Because if he doesn't have such a path, DeSantis doesn't, either.
The only problem with your scenario is that I don’t think the RINOs will back DeSantis either.
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If DeSantis is the candidate, Republican politicians will want Newsom to win to weaken America First Republicans. They want a pro war President and will prefer a Democrat who starts wars over Trump or DeSantis.