Hypothetically, if Constitutional law is being violated in an area where the Executive branch has jurisdiction, and the President refuses to enforce the law then that would be illegal.
Having said that, if the law as it's being interpreted, is not what the people want, then there is a legitimate process to rectify that.
Having laws on the books that are routinely ignored for political reasons is never a good thing. U.S. immigration policy is a good illustration of this.
I understand the rectification process and agree that laws "on the books," by congressional legisilation should be enforced.
But a President refusing to enforce a court decision is not unprecedented as I think you would agree.