Context is important. For the Republican party, that means decades of using "border security" as a dog whistle for keeping out Mexican immigrants.

For Trump in particular, that means his speeches as candidate, "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. And some, I assume, are good people." He is a fearmongerer, and this is just another step of playing along fears of immigrants.