"Why should I trade one tyrant 3000 miles away for three thousand tyrants one mile away? An elected legislature can trample a man's rights as easily as a king can."
--Benjamin Martin (The Patriot)
Although the Constitution is grounded in natural law, it is not a perfect reflection of it. As demonstrated by the Three Fifths Compromise, even slavery can be rationalized by a people claiming that liberty is an unalienable right.
Is there any hope that a governance system can properly reflect natural law? If the system takes a wrong turn, as it surely will given human tendencies to secure the strong arm of government for political benefit, who provides the last defense against violations of natural rights?
It is unlikely to come from the executive branch. Our founding ancestors well understood that presidents are more likely to usurp power for purposes of discretionary rule. But those limitations haven't worked. Lincoln, for example, was on the record as supporting slavery and signed off on the Corwin Amendment designed to perpetuate it.
It is unlikely to come from the legislative branch. Our founding ancestors also realized that legislators can easily trample individual rights and therefore shackled Congress with expressed limitations. But, again, those limitations haven't worked. Congress, for example, passed the Corwin Amendment designed to perpetuate slavery by supermajority vote.
What about the judicial branch? Ah, yes, the courts. Judges should be the ones. If individual rights are being trampled by tyranny, whether those tyrants are few or many, then the bench, well-versed in natural law, would rescue liberty. But that hasn't worked either. History has demonstrated that the basis for rulings, even those of the high court, is not natural law. Instead court rulings generally reflect positivism. In the Dred Scott decision, for example, the Supreme Court ignored the rights of man and upheld the institution of slavery, claiming that it was law and required a constitutional amendment to overturn. Of course, Jefferson and the antifederalists foresaw the problems with judicial review.
What is the last line of defense for liberty when no branches of government act on the basis of natural law?