There is a law that defines those born in the United States as citizens. Two actually. The 14th amendment and the very first provision of section 1401, which merely repeats the 14th amendment's provision. Congress only has the power to define rules/laws for naturalization. It's right there in The Constitution's Article I, Section. 8.
The Congress shall have Power...To establish an uniform Rule of Naturalization
But there is no power to define who is "Natural Born".
Have I pinged you to what they're trying to do in Missouri with @HOUSE BILL NO. 1046
You ought to get a real kick out of that.