Or hoping he somehow gets disqualified, or says the wrong thing and makes his rabid base turn on him
Or maybe they're hoping he'll flee to Russia
Basically, Trump is playing a dangerous game, and there's several ways it could go wrong. None of these guys can beat Trump, but if Trump goes down for any other reason they want to be waiting in the wings as second-best in the eyes of his followers. But in the meantime they don't want to just get completely consumed by him, and they want to appeal to moderates.
If Trump perceives them as a threat, he'll turn on them.
If they try to ride Trump's coattails, he'll use them and then abandon them like everyone else.
Their best bet is to not directly confront Trump but also not fall under his shadow. Supportive, but separate.