1) There is now a market for whether or not Trump will drop out before August 31. It is trading at about 8% on PredictIt. There is not much traffic yet, and this type of long-shot contract is a little unreliable. But, I will keep a close eye on it.
2) The second thing to consider is difference between raw market prices for Trump and GOP. Of course, people may assume that if Trump drops out the GOP is doomed. That a replacement will have no time to build a campaign and win. PredictIt has no difference in raw prices, but Betfair is giving GOP about 1-2 pennies better action.
My thoughts are that the probability is low, but I will certainly keep a close eye on this …