Good point by Max, also from the old authoritarian playbook. If you can't deal with the actual problem, attack the usual suspects to distract from the problem.
It's also an example of fighting "the good war" forever. E.g. how Putin is eternally celebrating WWII, the last time Russia was on the right side of practically anything (after first allying with Hitler, of course).
Immigrants to Trump and his fans are just a punching bag to be pummeled at every opportunity, especially when distraction is needed. When the audience gets restless, play the greatest hits.
