Could it be Cruz after all?
If Trump wins 1,237 or more delegates -- a majority -- he wins the nomination. On the first ballot at the convention, delegates are "bound" to vote for the candidate who won them in their state's primary, caucus, or other process, no matter what they themselves prefer. This is what usually happens -- one candidate gets a majority before the convention, and the first ballot at the convention is just a formality making his nomination official. (There are some additional complications involving "unpledged" delegates not bound to a candidate, delegates bound to candidates who have dropped out, etc., but the point is, normally one candidate has an absolute majority and thus wins.)
But if no candidate gets a majority on the first ballot, further votes are held until someone does get a majority -- and in those later votes, most of the delegates (who, remember, are actual people and not just numbers on a scoreboard) are free to vote as they choose, regardless of whom they were originally "bound" to. This is where Cruz's current strategy comes in.
Actual delegates are chosen in the various states a few weeks after those states' primaries or caucuses. Cruz's people have been hard at work in state after state, packing the delegate slates with their man's supporters. Those of them that are "bound" as Trump delegates will have to vote for Trump on the first ballot at the convention -- but if he fails to reach the magic 1,237, there will be a second ballot on which they'll be free to vote for Cruz.
(It's even possible that, even if Trump has a majority of "bound" delegates, he could be denied a first-ballot win if some of "his" delegates break the rules and vote against him. What would happen in that situation? From what I've seen, the answers are murky at best.)
Why hasn't Trump's campaign been similarly engaged, ensuring that his delegates are genuine, committed Trumpanzees? Most likely Trump has been assuming he would get a majority on the first ballot, and in any case his slipshod and amateurish campaign has repeatedly shown an inability to understand and deal with the more Byzantine nuances of the process.
If Cruz takes the nomination on the second ballot, thwarting both Trump and the bosses who had planned to impose a hand-picked nominee like Ryan or Romney, the party will be left even more divided and confused than if the bosses had won. The Trumpanzees (and Trump himself) will be just as outraged at the nomination being "stolen", while the leaders may well balk at uniting behind a man whom they loathe and who cannot win in November. They could end up split into three hostile factions instead of two.
Cruz, in his way, is just as dangerous as Trump, and just as doomed in November. He's simply this cycle's Christian Right candidate, the avatar of the Bible-thumping, snake-handling, tongue-speaking element which has been growing in influence within the party for years. He doesn't play well outside of that constituency; much of the support he's attracting now is really anyone-but-Trump sentiment. If he were the nominee, the rumors of extramarital affairs, and questions about his Constitutional eligibility as a natural-born citizen (he was born in Canada and his father was not a US citizen at the time; it's unclear whether his mother was), would get heightened attention. So would his string of endorsements by outrageously bigoted preachers who support imposing the Biblical death penalty for homosexuality. Then there's his father's deranged Dominionist ranting. Dildo jokes would proliferate. This is not a formula for winning in November, nor is it the image that Republicans outside the lunatic-theocrat faction want to project. But they may be stuck with it.
[Image at top (Satan preserve us) found via Progressive Eruptions.]