Such a scam issue. Trump uses it as a dog whistle because he knows it riles up a select portion of gullible white people who are terrified of immigrants that just want to clean hotels, pick fruit, and nanny some fat, spoiled American kids (oh noez!)
The fact of the matter is most of these folkes are actually quite conservative, they are quite religious usually, anti-abortion, and pro-business/pro-entrepreneurship. They're probably more conservative than the average American. If Republicans would stop demonizing them every 10 seconds they might actually get some of those votes.
In the long run they will have to, the "Trump crowd" is mostly old, bitter people who will be dead in 10-15 years. They can't rely on that base forever, the younger demographics of the US are much more diverse and they will have to eventually accept that the politics of scapegoating groups like Latinos won't work.
A lot of it is bunk too, in the 17 years since 9/11, how many terrorists have come from the Southern border? Zero. If you wanted to be a gang-banger criminal, which is a better place, Mexico, where you can do as you please, or the US where a person of color can get their ass thrown in jail just for looking sideways at a cop and has a for-profit prison industry based on throwing tons of people into jail?
Trump has also flip-flopped on a lot of bull shit, he was pro-choice and even backed Bill Clinton's assault weapons ban in the 90s and 2000s ... good luck getting him to admit to that today. Also I'm sure he had his clothing line made in China just by accident too.