Since ancient times, some people have thought there was a natural law based upon principles that were superior to man-made law. Natural law is based in reason. It asserts a permanent morality which applies universally. Thomas Jefferson sought to define some features of it in his declaration of independence.
For myself, after Darwin, I think there are natural instincts above these: by natural instincts, I mean particularly the desire to survive.
That's what we are seeing among Republicans in congress. They are a herd that has been driven together. Unfortunately for them, I suspect the herd is heading over a cliff.