What do you mean by "republican?" What do you mean by "fear?"
My opinion: the GOP establishment is just peachy with Mrs Clinton, and always have been. They'd have been happier if one of their own proposing could have won, but Mrs Clinton is by far the lesser of evils for them than anyone left in the field. Well, maybe Mr Rubio, who is more easily controlled. But still, what do you mean by "fear?" Afraid she'll win, or afraid what will happen if she wins? They have no fears on the latter score; they hope she will win if Mr Trump is the other candidate, which seems likely.
The GOP base? They don't fear Mrs Clinton, they hate her. She's a female. She's an aristocrat. She has some liberal social views. They will turn out in droves to oppose her, but those droves probably won't amount to much in the end.
All-in-all, IMO, the cleavage between the GOP establishment and the GOP base is quite simple: the establishment really don't care at all about social/culture wars, they care only about power and feeding at the public trough. They care about economics, and its handmaiden, foreign policy. Their only interest in domestic questions is throwing chum into the waters to distract the little fish, who for their part are only interested in social/culture wars subjects, and have little, if any, opinion on economic issues except they don't want taxes raised. As for foreign policy, the only foreign policy Joe Sixpack understands or cares about is bombing people into the stone age. U-S-A, U-S-A! Ironically, Mrs Clinton would be just as satisfactory on that issue as any of the Republicans.
-- Mal