An American Immigration Policy.
The point is not that those South of the border are bad people. Many of them are fine people--though certainly not all. But they have different priorities; different purposes in coming here; different aspirations. We on the other hand have limited space; limited resources (as does every people). And our first duty--our sworn duty--is to our own.
To better understand the unique perspective of the Founders--the perspective that the Left seeks to undermine-- consider: America: Based On Experience & Reason.