These things snowball.
At first there's the odd Republican criticising Trump. Then, as soon as they feel losing is more likely than winning, others will join. If he loses, the dissent will grow stronger and stronger and, inevitably, they'll reach the conclusion that really they lost because of Trump. Because if it's not Trump, it's them. And it can't be them. Then they'll use that to regroup and refocus the GOP and in doing so, they must create distance between loser Trump and themselves.
Only a very popular candidate is able to withstand these forces of political nature.
Trump is not popular. In fact, if it was up to the popular vote, we'd hardly need the elections. Everyone knows he'd lose. On top of that he's 78, overweight and in poor shape generally. If he dropped dead tomorrow, not a single physician on the face of the Earth would consider that remarkable on a medical level. The possibility of other Republicans betting on a 78 year old man (82 by the next elections!), who is not popular with the American people and has lost the presidential elections twice, is zero.
If Trump loses, he will suffer.