That's nonsense, there's no candidate for the Republican party that even hits double digit support from those that announced primary challenges.
Joe Walsh - Republicans hated him from the get-go and he's polled like garbage in the few polls I've seen. He's hardly a stand-up guy either, being the "YOU LIE" guy from the Obama SOTU
Bill Weld - Similarly, non-existent polling.
Rocky De La Fuente - Fucking who?
Mark Sanford - Similarly nonexistent polling, with a campaign kickoff that was apparently primarily attended by 1 local media reporter and a group of curious, confused French students who wanted to know why he had a gigantic check.
This is not an actual fear. The fear is insulting Trump or damaging his sensitive ego. There's no chance of any Republican primary challenger posing even the tiniest of threats to Trump. It's all about his ego.