Suppose Trump arrives at the convention with fewer than half the delegates and so doesn't win on the first ballot. Many delegates, including many of his, will be free to vote for someone else on the second ballot, and almost all will be free on the third ballot. His delegates are not selected by him but by the state party machinery; although they are obliged to vote for him on the first ballot, what they do thereafter will depend on the party's internal politics. It could get complicated, given that the obvious alternative at this point is Cruz, who is almost as unpopular with the Republican establishment as Trump. But my guess is that if the Republican leadership wants to deny Trump the nomination at that point, they can do so.
There are two reasons they might not. One is that they might think Trump is more likely to win the election than alternative candidates—that, according to Ben Carson, is one of the reasons he endorsed Trump. The other is fear that if Trump's supporters think the nomination has been unfairly stolen from him enough of them will stay home on election day to throw the presidential election, and possibly congressional elections as well, to the Democrats.
That raises the question of what counts as stealing the nomination—not in the eyes of God but in the eyes of Trump supporters. If Trump starts with almost enough votes and a lot of his delegates defect on the second ballot, that might do it. If there are changes in the rules that appear designed to keep Trump from winning, that might also do it.
The current rules, as revised for the 2012 convention, limit nomination to candidates who have a majority of the delegates in at least eight states. That would almost certainly eliminate everyone but Trump and Cruz, might limit it to only Trump. A change in that rule that let someone like Romney be nominated and win would be entirely legal, as I understand the rules. But it might not look that way to Trump supporters. A change that let in someone who ran, didn't do very well, but is more acceptable to the establishment than Cruz, might be seen in the same category.
Can Trump be elected? The obvious answer is "no," but I am not sure it is correct. He did a great deal better in the primaries than almost anyone expected. His claim is that he can get support from Democratic voters. If true, the question will be whether he gains more votes by attracting Democrats than he loses by repelling Republicans.
My explanation of why Trump and Cruz are the two Republican candidates still standing ties into my old explanation of why people vote. Also my explanation of why sports teams are linked to cities
Part of the reason to attend a football game is
the pleasure of partisanship, of cheering for your team. Identifying a team with a city or a university provides a
preexisting body of partisans. Every four years a game is played out
across the nation with the fate of the world at stake. You can not
merely cheer for your team, you can play on it, even if in a very minor
role, for the cost of an hour or so of your time. Who could resist?
Apply the same approach to the nomination campaign. The decision of who to support is based not on who supports what policies but on whose team you enjoy
identifying with. You wants to be on a strong team, a team with a confident, aggressive leader. Trump and Cruz fit that pattern considerably better than Bush and Rubio. Trump's repeated references to "little Marco" were entirely unfair—but effective. Being physically big fits the image of a strong leader.
I watched the beginning of the most recent Republican debate and thought Trump did a clever job of deflecting, arguably reversing, what should have been an effective attack against him. He claims that H1B visas and free trade agreements steal jobs from American workers. But he himself brought in foreign workers on H1B visas to work at his companies, outsourced parts of what he was doing to foreign countries.
The obvious response would be to deny it or claim the charges were exaggerated. Trump's response instead was that, as a businessman, it was his job to make money for his firms within the existing rules, good or bad. He went on to argue that, because he had experience taking advantage of bad rules, he knew how to stop other people from doing it—and would. "Set a thief to catch a thief." Or, alternatively, when presented with lemons ... .