This is basically a clash of two schools of thought;
Those who still see the issues internally within first world states, and believe we should use our vast moneys and resources to cater exclusively to the problems already there, and those who see the first world states as a good elite that can use its moneys and resources to help under developed countries and its peoples.
The issue is that over the last decade/two decades there has been a lot of cultural transplantism (mistakely conflated with multiculturalism) within certain first world states which has resulted in strain and cultural clashes on a very low scale for the time being, and could easily explode into full blown race riots and other nasty issues should things go just slightly the wrong way. British apathy and faith by the majority in the legal system has kept a decent enough lid on this, and the government is currently working on how to better integrate the more stubborn communities with language lessons and other assistance.
The United States is highly fortunate in that, for the most part, it is well isolated from the majority of the world, holding the majority of arable territory of the North American Continent. To the North is a very rich neighbour with a nice, small and stable population and to the south is a source of potentially cheap labour that will take up the jobs that "true americans" are too good for. Which has always been the traditional lot of migrants.
The bigger, underlying issue, is the fact that the majority of US heavy industry is starting to die off, favouring factories in places such as the far east where production is simply cheaper, those who might want to move into a different sector (albeit temporarily) are finding Mexicans and other minorities within those jobs already, with enough of a demand for those jobs (at the right price) being filled by more who cross the border by fair means or foul.
The nativism appeal comes about by Trump actually targeting other groups, minorities and illegal migrants which, while founded in some truth, are typically overblown to a far greater proportion than they actually are as an issue.
The isolated racist incidents at his rallies don't actually delve into Trump's nativist policies, which includes favouring native born citizens (regardless of ethnicity) over newly arrived migrants for things such as further education, grants, financial assistance and jobs. His plans on eradicating ghettos sounds a little sinister on paper, there was talk of basically moving around the poor inner city african-american minotiries to places of better jobs and ensuring there would be a disproportionate grant-based education in favour of such candidates. There's a certain, broader appeal to that than the sort of indifferent shrugging that has gone on from the Obama administration regarding the US-Mexican border.
