Trump is a fascist, and he needs a dark and dangerous enemy to scare the American people with, and he cannot use American African Americans, so immigrants are the target.
Truth be told, the best way to secure the borders from undocumented migration, if indeed that is what this country wants, is to penalize employers, not harass the poor and powerless workers.
America has had, for years, a porous border, for economic reasons...it needed the workers. Now, it is telling people who have been in America for decades that they have to leave.
Americans deserve to have strong border policy, but both political parties let things slide.