Basically, we can trivially tap into the human mind's natural inclination for tribalism and use undocumented people as a bargaining chip. We can tell people that THEY cause this problem and that problem, because THEY don't share our values, our skin, our culture, our mindset.
It works extremely effectively because:
1. Everyone likes to hear that they're not the ones causing problems, someone else is.
2. Humans naturally segregate based on commonalities. So people are in one group, and undocumented people in another. Sure seems like us versus them then.
3. The solutions are simple, so people have high confidence in them. Build a wall. So simple and easy, a toddler can understand that! Whether it works is not relevant.
4. Americans already have a strong sense of national identity.
5. The economy and perceived economy/QOL is actually in trouble, so we have legitimate justifications for our populist messaging.
But, had it not been undocumented people, it would've just been something else. We know that because we do see it successfully, time and time and time again, for other groups.
Trans people. That's another one that is in use today, right now, for populist messaging and it's highly effective.
I thought there were like 1 million max.