Trump is popular because his policy positions fit Demographics of the GOP base. These are older middle class people who are afraid of competition from immigrants and terrified that their Social Security will run out. They are Right Wing on Immigration, but Left Wing on their own Government benefits. They do not view this as contradictory. In fact, this used to be a perfectly mainstream Progressive stance. So the message is seductive to parts of the Democratic base as well.
If Trump were less of an asshole he would be even more popular. His antics attract attention, but his supporters stick with him for solid reasons. They view him as representing their interests.
What they don't understand is that it is the immigrants they want to deport are essential to keep Social Security solvent. Without immigration the US would be just another rich country with an aging population. It would not be able to sustain the generous benefits it gives to older citizens. It is a myth that the Social Security is paid out to you by the Government from what you paid into the fund. Most people getting SS now are getting many times what they paid in, including interest. In fact, people currently working are paying for those who have retired. So without younger workers (legal or not) the system would collapse.
The fertility rate in the US is 1.86 children per woman, well below the replacement rate of 2.1.
It is lower than that of France, Australia or the UK. And yet the US has a more rapidly growing population (+0.75% annually) than other rich countries. The difference is made up by immigration.
The proportion of Non-Hispanic White people in the US today is 62.6%. It has shrunk from 75.6% in 1990. The fertility rate among them is even lower, 1.76 children per woman. The median age is 42.3 (compared to 27.6 among hispanics). This older population is sustainable only by the large numbers of immigrants and their children entering the workforce. These trends will continue.
So, that brown child that the Trump supporter wants to deport will be paying for his SS some day. Who will explain this to them? Certainly not Trump. Not Jeb!, who has joined in on the race-baiting ("anchor babies"). Not Jindal, who would cancel his own birthright citizenship. Not Cruz, who might yet become Prime Minister of Canada. Not Huckerbee, who expects to depopulate the US not only by deportation, but by the Rapture.
It falls to the Democrats to explain this Arithmetic. Bill Clinton said in his 2012 convention address that Arithmetic will beat the Republicans. It could defeat also Trump's emerging Third Party.