The party leaders dislike Trump. It may be because he offends their principles; because of his vulgarity and lack of decorum; or simply because they don't think he can win. It's likely to be a combination of all three.
It's within their power to deny Trump the nomination. If he doesn't have an outright majority on the first ballot, most of the delegates are released to vote for anyone they want. Even if he wins a majority in the primaries, the party can change the rules before the convention. It would look bad, and Trump and his supporters would understandably be enraged, but they could do it.
They would need a not-Trump candidate to put forward. Cruz is in a strong second place in the delegate count, but his Republican colleagues despise him to an even greater degree than Trump. Mitt Romney or John Kasich would be a plausible choice.
The consequences would be severe. Trump has threatened violence from his followers if he is denied the nomination. I would not put it past him to encourage his angry and heavily-armed supporters to form pro-Trump militias.
In many states it would be too late for Trump, or anyone else, to get on the ballot as an independent; but he could campaign as a write-in candidate, and call on his supporters to reject the Republican party. In all likelihood, the GOP would not only lose the presidency, but be hamstrung in congressional and local races as well.
Afterwards, Trump would not go away. His foray into politics may have started as an exercise in self-promotion, but he seems to have a taste for it now. If he is denied the Republican nomination, his pride will be wounded. He would very likely spend the next four years plotting revenge, organising a third party to support him, and perhaps backing it up with those armed militias. It's a terrifying prospect for the entire world, and in particular for the Republican party.
A deal with the Devil
The alternative is to cut a deal with Trump. At the moment, the party has something he wants: The Republican nomination for President. The write-in candidate scenario is not optimal from Trump's point of view either. He would much rather take up the nomination smoothly, and concentrate on Hillary Clinton in the general election. He would prefer the grudging support of of the GOP to its active hostility.
So, in exchange for concessions on his choice of running mate, planned Cabinet appointments, and the like, the party agrees not to deploy shenanigans, and Trump becomes the nominee. He picks some relatively mainstream Republican for vice-president, such as Trump endorser and blank-eyed hostage Chris Christie. The GOP prepares itself for likely defeat at the hands of Hillary Clinton, and begin planning for 2018 and 2020.
To a cynical Republican operative, this may look like the lesser of two evils. There's just one problem: It requires them to endorse an openly fascist candidate.
From the convention in June to the election in November, Trump would have an unrivalled platform from which to spew his bigotry and incoherent rage. He would be doing it with the support of one of the USA's two main political parties.
The Republican party has done despicable things, but always cloaked in respect for the forms of constitutional government. Trump throws all of that away. If he is the nominee, the Republican party will have embraced his contempt for the rule of law. Intimidated by Trump's endorsement of mob violence, they will have decided to join him instead of fighting him.
If Trump lost badly in November, he might not run again, but there would be no putting the beast back in its cage. Trump's brand of fascism would have been legitimised. Ambitious younger politicians would try to position themselves as Trump's heir. The consequences could be worse than anything we have yet seen.
Not only that: Trump could win. He's a vulgar and bigoted buffoon, but his message has a following. He speaks to those who are angry and disaffected with the political establishment; and Hillary Clinton is the quintessential establishment candidate. The unaccountable powers of the President over intelligence and the armed forces, extended in the name of homeland security and the war on terror, could fall into the hands of Donald Trump. The Republicans might imagine that Congress would be able to contain him, but I would not be so sure.
It may be that Republican leaders are ready to fight Trump at the convention. Paul Ryan, Speaker of the House, has remarked that he is preparing for a contested convention, which is a mildly encouraging sign. On the one side are the long-term health of the Republican party; whatever remains of its political principles; and the well-being of the United States of America. On the other is cynical, short-term tactical advantage.
It is not a foregone conclusion.