President Trump talks a lot about undocumented immigrants and does horrible things to them including: ripping children from their parents and putting them in cages, and trying to deport Dreamers and their parents.
But while he uses his executive power to reek havoc on undocumented immigrants, his legislative fight is very focused on legal immigration, and so is the next stage of executive action. He wants to basically close our borders. He wants to make it so hard to come to the United States, the best and brightest scientists, the most ambitious young families, the people who have made the United States such a dominant force in the world will stop topping off our talent pool. In short, he wants to hack legal immigration by half. He wants to make life so miserable for LEGAL immigrants, already here, that they leave too.
Cruelty: beyond the immigrants who will simply not be able, or want to come to the United States, this policy will make life risky and unpleasant for legal immigrants. Expect to see a zero tolerance policy for legal immigrants as the government will start deporting legal immigrants for anything from traffic stops to non-violent drug crimes. As the United States moves to deport legal immigrants over anything, families will be ripped apart, business interest destroyed, making the United States an insecure place for bright and ambitious people to invest.
1) Economics: we benefit from the best and brightest from around the world coming to the United States universities and technology firms. We also benefit from ambitious people of lower skill, who power growth. Higher education and technology are two industries that are already hit hard by the Trump administration's tightening visa restrictions and (court-blocked) Muslim Travel Ban. Many top students who would have come to the United States, worked with our scientists, trained our students, and stayed on to build thing here, will go elsewhere.
2) Economics of Native-born Americans: it is not just that the economy suffers overall, but all Americans pay that price. There is no evidence that immigrants take jobs from native-born Americans, as their economic impact of increased growth dwarfs any such effects. If you care about inequality you should support progressive taxation, meaningful regulation to create safe and productive work environments, s social safety net so people can focus on productivity, not survival, and unions which help counter market power and asymmetric information that allows big corporations to pay workers less than marginal product.
3) Security: we benefit from diverse population that trust and works with our government, rather than resents and fears it. We need people from immigrant communities to work with authorities, but if authorities are in a constant state of opposition and disruption with immigrant communities, there will be no cooperation here (or abroad). This will weaken our security much more than any risk from having immigrants in the country. Side-note: right now "Justice" Department admits that it has reallocated resources from fighting drug crimes to help cover President Trump's efforts to rip children from parents.