Will the Republican Establishment walk on a Trump nomination, should he win? If it does, let it walk, as it did in 1964. What the Trump phenomenon represents, whether the Washington establishment is appalled or not, is the future. Take a look at Europe. Ethno-nationalism from Scotland to Catalonia to Flanders, and nationalism in the form of parties like the UKIP in Britain and FN in France, new governments in Warsaw and Budapest — this looks more like the future than Angela Merkel or the EU.
A party will not survive that yields to an establishment ultimatum that — either you accept our choice, or we walk. The answer to that is: Go ahead and walk!