Even if he goes without a fight, Trump may have changed the GOP forever by exposing the vast gap between white working-class Republican voters and the party leadership. The legacy of his campaign may be a split into two parties, or at least two distinct factions—one nationalistic, nativist, and antitrade; the other internationalist, pro-immigration, and pro-corporate.
More immediately, Republicans face a possible Samson scenario in which Trump tears the party down around him. This is the course he has implicitly threatened by warning of riots if he arrives in Cleveland with the most delegates and is denied the nomination. It would not take much for him to instigate violence at the convention. The rabble that he has roused seems to crave it as a forbidden form of political expression. A rebellion will be harder to pull off if the party comes together around Cruz on a second ballot, as opposed to a backroom deal to nominate House Speaker Paul Ryan or another candidate who didn’t run in the primaries.
The only thing worse for the GOP than chaos in Cleveland would be a third-party campaign by a vindictive Trump. If he waits until the convention, it would be hard for him to secure ballot access as an independent in several key states. But even as a write-in candidate, he would draw significant votes from the Republican nominee, ensuring a Democratic victory. If rejection propels Trump to play spoiler for his party, he has the means to do so.