Longtime Republicans are expressing a growing fear that the coming election could be shattering for the party, or reshape it in ways that leave it unrecognizable. Rank-and-file conservatives, after decades of deferring to party elites, are trying to stage what is effectively a people’s coup by selecting a standard-bearer who is not the preferred candidate of wealthy donors and elected officials. And many of those traditional power brokers, in turn, are deeply uncomfortable and even hostile to Mr. Trump and Mr. Cruz: Between them, the leading candidates do not have the backing of a single senator or governor. The splits within the party would be difficult to heal no matter the nominee.