Ramesh, I’m inclined to think that the tradition of the executive defending validly enacted federal laws is a good one, but it has limits. The Solicitor General’s office acts as the federal government’s lawyer and has an obligation to zealously represent its client. This means the SG should make any and all arguments in defense of a law’s constitutionality that can be made in good faith. The difficult question arises when the executive determines that there is no plausible basis upon which to defend a law’s constitutionality. In such a rare instance, the executive probably has an obligation to demur. If Congress does its job, however, this should be rare, so the tradition should hold.

Of note, there is a prominent example of the executive refusing to defend a law’s constitutionality during the Clinton Administration. In the Dickerson case, the Justice Department refused to defend the federal law that limited the Supreme Court’s Miranda holding. This forced the Court to appoint Professor Paul Cassell (now a federal district court judge) to defend the law’s constitutionality. I believe this was a violation of the tradition, as the law in question was quite defensible on constitutional grounds.

P.S. SA’s Feddie has more thoughts on the subject here.