Those officials who would throw our borders wide-open like to call opponents to their open immigration policies racists, or at least profoundly unenlightened rednecks. No doubt there are some of those out there. But that's beside the point.
The issue is that it is not possible to properly vet these migrants and refugees, meaning you will allow potentially thousands of individuals into the US who are intent on committing criminal acts. This danger may not be a concern for these government officials, many of whom have very comprehensive, taxpayer-funded, 24-hour security teams. But for the average American, it is an issue of personal safety. And the news of violence by immigrants only supports their concerns.
What news? See page two.