I know it is fashionable to day "Don't underestimate Trump. We must take him seriously."
BA-LO-NEY !!!
Trump cannot win a general election. There are almost no Dems would would vote for him and a big part of the GOP base will not support him. If nominated, Trump sets back the GOP efforts to court the Latino base by 10 years. There are no scenarios where the numbers add up at all. This would be a Goldwater election.
Worse than that (for the GOP), having Trump on the ballot would bring out Dem voters in HUGE numbers -- HUGE!!! And this would help up and down the ballot.
Trump is a GOP nightmare and the GOP knows it. The GOP would love to get rid of the guy, but Trump is enough of an asshole to run a third party campaign, which would guarantee a Dem victory for the White House. However, considering the big picture, that is probably the GOP's best option. They cannot allow him to be their candidate.
If the polls were closer, then the GOP could use their local apparatus to block Trump from winning enough Primary delegates. And in that scenario, they could buy him off to step aside. This is probably why Trump ran in the first place, to blackmail the GOP for favors worth billions to him. But a funny thing happened on the way to the forum. Trump jumped to the top of the polls and is staying there. If Trump really believes he can be President, then there are no deals the GOP can offer him that are as good as the deals he can wrangle as President.
Trump cannot win. But he believes he can, and that's what is driving things right now.