It is true that Trump is a loudmouth, racist bigot. Many think that this is his appeal - that there is a big pool of racist bigots, especially on the right of the political spectrum, who have found in him a champion of their cause. Perhaps there is some truth in this, but I don't think this pool is big enough to explain Trump's success. Conservative commentator (and former independent presidential candidate) Pat Buchanan has I think identified three reasons why Trump is so popular, especially among white working class Americans.
Buchanan suggests that the Trump phenomenon is a rejection of what he calls "Bush Republicanism" i.e. the policies of the Republican party over the past quarter century (the time of the Bushes, pere and fils). Three aspects of GOP policy, during this era, have been particularly inimical to the interests of conservative-minded working Americans. They concern:
1. Free trade.
3. Foreign wars.
While there are many advantages to free trade (and economic theory to justify its benefits, going all the way back to David Ricardo in the early nineteenth century) it is often overlooked that there are losers as well as winners from free-trade agreements. I think it is pretty much indisputable that the working classes of the USA, Britain and other early industrializers have been among the biggest groups of losers. Well-paid manufacturing jobs have all but vanished from many once prosperous manufacturing regions. With this collapse of industrial jobs has gone the weakening of unions. There have indeed been many winners from free trade, not least of whom are consumers, a group that overlaps considerably with industrial workers. But cheap goods for consumption are not much use if you don't have an income from a decent job. But on the American side the biggest winners from free-trade have been corporations and the owners of capital, who are able to outsource production to low-wage countries.
Immigration, again, is a two-edged sword. It has undoubtedly helped many people from poor countries and it has provided a willing workforce, especially for jobs which many Americans don't want to do. Immigration has also enriched the cultural life of the host countries - think what a Sunday in Toronto was like in the Fifties. But at the same time, immigration has helped reduce the bargaining power of labour, because many immigrants (especially illegal ones) have been willing to work for much lower wages than native Americans.
These two aspects of globalization have been embraced not only by the Republican Party, but also by the Democrats. But the GOP can claim almost exclusive ownership of the third issue - foreign wars. The US has been almost constantly at war since Bush Junior's ill-advised and illegal invasion of Iraq in 2001. There is no end in sight. The costs of these wars has been enormous. Funding these wars while cutting taxes, especially for top income earners, has led to large budget deficits and an ever-increasing debt burden.
While all three of these GOP policies have undoubtedly hurt many ordinary conservative-minded working Americans, the irony is that a large number of them have been deceived into supporting the party - a party which really does not represent their interests. The GOP has achieved this by appealing to wedge issues like abortion, religion etc. and patriotic nationalism. At the same time they have badmouthed their opponents as "liberal' elitists and internationalists. But when in power Republicans (both federally and at the state level) have done everything they can to break unions, reduce taxes on the richest, and contract out work to outfits which pay minimum wages (often to illegal immigrants) - all policies which benefit corporations and 'capital' over 'labour'.
I am not sure that it is even possible or even desirable to reverse some of the policies on free trade and immigration. Stopping the foreign wars however would be an unalloyed good. Trump has stood for a reversal of all three of these policy choices made by the Republican Party and I think this is probably the main reason for his popularity. It seems that many ordinary Americans, especially of the working class, are waking up to the fact they they have been sold down the river. They want no more of free trade - witness the opposition to the TPP - and a slowing down, if not a complete stop, to immigration, especially illegal immigration. Trump too has voiced his opposition to perpetual Middle Eastern wars.
Trump's advantage is that he is independently wealthy and is not beholden to the big money players of the Republican Party. So he can gladly refute the policies which have made some of these players very wealthy. Also he feels he doesn't need backing from AIPAC and other pro-Israel groups, so he can steer his own course on the Middle East.
So it seems to me that many Americans are waking up to the fact that, over the past couple of decades, they have been had. Many, who incline to the right, don't want to nominate someone like Rubio or Cruz, which would lead to much more of the same (or worse especially in the case of Cruz). I believe Hillary would be in the same mould too - probably worse with respect to foreign wars.
Trump is an unpleasant, bigoted and racist blowhard. He is a loudmouth braggart, with no experience of government. The thought of having him in the White House is truly frightening. But I think one has to admit that he has touched a chord, and that he has positions which genuinely represent the interests of a large group of Americans, even if he expresses them in a brutish way. Perhaps, through doing this, in the long run, he will turn out to have changed the course of American politics.