Donald Trump's signature issue is immigration. When asked by the New York Times what he would accomplish during his first 100 days as President, Trump responded: "rescind Obama's executive orders on immigration," design the wall with Mexico, and ensure "the immigration ban on Muslims would be in place." Trump's immigration policy has five pillars.
Immigrants are dangerous: A January NBC News poll found that 34 percent of Republicans thought "terrorism" was the biggest issue facing the US; another 13 percent said it was "immigration." Trump has linked these two issues and staked out a position so extreme it outflanked the other GOP contenders, making him the presumptive Republican nominee.
In his June 15th announcement speech Trump said:
The U.S. has become a dumping ground for everybody else's problems... When Mexico sends its people, they're not sending their best... They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists.
On July 5th, Trump asserted that Mexicans were responsible for "tremendous infectious disease... pouring across the border." Trump's first TV ad implied that ISIS fighters were also "pouring across the border."
Immigrants take away jobs: In a July 11th speech, Trump made further claims about immigrants: "They're taking our jobs. They're taking our manufacturing jobs. They're taking our money."
According to an August Rasmussen poll 51 percent of Americans "believe illegal immigrants are taking jobs away from American citizens."
However, an August Forbes magazine article said this belief is incorrect: "illegal immigrants actually raise wages for documented/native workers."
Immigration can be stopped by building a wall along the Mexican border:: Trump promises to build a wall along the open border with Mexico. When pressed, Trump said the wall would be 1000 miles long, rise 35-40 feet, and cost $8 billion.
The Washington Post studied Trump's wall design and estimated that it would cost $25 billion for design and material; in addition, the construction would require "40,000 workers per year for at least four years."
Trump insists Mexico must pay for this wall. If they do not, he promises Mexican citizens will be subject to penalties on remittance payments, tariffs on temporary visas, and increased fees on border-crossing cards and at ports-of-entry. Legal experts believe that Trump's reimbursement scheme is illegal.
However, the bigger concern is whether such a wall, if built, would accomplish its objectives. Politifact noted that, in recent years, there has been zero net immigration across the Mexican border - that is, the number of folks going north is matched by the number of people going south. (This was confirmed in a November Pew Research report.)
Pew Research says there are actually 11.3 million illegal immigrants (that comprise about 5.1 percent of the US labor force). The Atlantic estimates it would cost $140 billion to deport them - with additional billions in economic consequences.
Trump blames the existence of illegal immigrants on President Obama's unwillingness to enforce immigration law. Politifact says this is an incorrect statement.
Ban Muslims from entering the US: On December 7th, Trump called for "a total and complete shutdown of Muslims entering the United States." Separate from enforcement expenses, it's estimated this ban would cost the US $18.4 billion in annual lost tourism.
There are many other problems with banning Muslims. In his recent foreign policy speech Trump promised, "We're going to be working very closely with our allies in the Muslim world." He didn't reconcile with this with his plan to bar Muslims. The countries with the largest Muslim populations are Indonesia, Pakistan, and India; all US allies. All these countries import weapons from the US, as do other Muslim countries such as Turkey and Saudi Arabia.
In September, Pew Research observed: "Republicans more likely to say Immigrants have a negative impact on U.S. society, crime and economy." (53 percent of Republicans say immigrants make U.S. society worse versus 55 percent of Democrats who say immigrants make it better.)
For several years differences between the brains of liberals and conservatives has been a hotly debated topic. A 2013 Mother Jones article observed: "fearful people are more conservative."
Whether because of fear, or some other reason, Republicans tend to have negative feelings about immigrants. Donald Trump has played to this and his pandering has made him the presumptive Republican nominee.