Trump's disastrous candidacy (disastrous for the GOP and conservatism) is all the fault of the GOP establishment, pure and simple. The last Republican nominee who actually seemed to give a crap about the white working class was Ronald Reagan (who even got the endorsement of the Teamsters twice!) but once the blue-blooded country club Republican George H.W. became President, the white working class within the GOP was ignored or outright suppressed.
Their core beliefs were constantly denigrated and they were called all sorts of names, bigot or racist being the worst. They were sick of minorities getting preferential treatment in hiring and college admissions. They were sick of losing their jobs to off-the-book illegal immigrants who can always work for less as there are no taxes associated with hiring them. They were sick of their factories shutting down so that employers could make a killing having their products made by people working at slave wages. They were sick of billions in foreign aid would go to those very same countries (e.g. the $50 billion bailout package given to Mexico just one year after NAFTA went into force).
These white working class Americans found a champion in Patrick Buchanan, who could speak their language as he cared about what they cared about and, importantly, understood them thanks to his own blue collar roots. Unfortunately, instead of making substantive responses to Buchanan and the Buchanan Brigades, the establishment carpet bombed them with ad hominem attacks. They essentially tried to shame this large part of the electorate to death. It worked for a while, even as there were fewer and fewer manufacturing jobs and these white working class Americans stood a better chance of finding a job at a Chick-fil-a rather than actually building something. W didn't speak for them as he was just a born-again blue blood. McCain was focused almost exclusively on foreign policy and campaign finance reform and Mitt had laid off scores of workers while at Bain.
The GOP establishment liked focusing on "small business owners", but there are comparatively few of them and many of the working class knew the GOP only mentioned them in arguments that attempted to lower tax rates on the rich, people they really don't give a crap about. Notice how every election year every GOP candidate has to come out with a tax plan even though taxes are low on the list of what the white working class cares about. No, they don't want to pay 50% of their pay in taxes but they generally aren't with effective tax rates that are often under 10% for people in their income range. They care about jobs and what the GOP will do about it. And they don't want to hear how changes in regulations etc. will eventually bring jobs back. These people are living paycheck to paycheck and need relief NOW! If the establishment hadn't been ignoring and suppressing these people for the last 30 years, maybe they would have more cushion so they could be more patient. But the cushion is gone and the patience is gone.
They are with Trump because he is a man of action and believe that if he comes to power he might actually force factories to come back to their area so they can have real jobs again. He will shut off the tidal wave of off-the-book labor that is constantly streaming over the border, costing them jobs. He is not a conservative ideologue like Buchanan but at this point they are too desperate to care.
So now the GOP faces complete and utter disaster. I have no doubt that Trump can win the general, that's not why I think it will be a disaster. He'll be a disaster because the vast majority of his beliefs are non-conservative and will lead many Republicans to question what's the point of voting Republican if this is what we get? What the point if the best we can do is nominate a national socialist to face off with an international socialist?
Actions have consequences and the GOP establishment calling their own voters bigots, racists and isolationists is having some very negative repercussions both for the party and the nation as a whole.
Let's all pray that Ted Cruz can pull it off on Saturday, he might be our last hope.