For those interested in the substance of what we do in legal scholarship, rather than professorial one-upsmanship, Robert Delahunty and I have published an academic paper criticizing President Obama's declaration in July that he would not enforce the immigration laws against 800,000-1.7 million young illegal aliens. We argue that there are cases where Presidents can decline to enforce a federal law, such as when a law is unconstitutional, infringes on presidential constitutional powers, or Congress fails to provide any resources. But Obama's declaration, we argue, violated the Constitution because Obama is refusing to enforce a law because he simply disagrees with the policy.
If that is possible, then President Romney could effectively kill Obamacare by refusing to enforce it, or lower the tax rate by only enforcing the tax laws at his preferred rates. Take a look at our argument here.