Ramesh, the theory is, exactly as you say, to remove politics from the process, but I shouldn’t have suggested it is an immutable rule. It’s a default position that can be varied if the circumstances warrant. Ed Whelan surely knows how this works better than I do — I happen to have gotten something of an education in it because I was one of a handful of prosecutors who went to Main Justice to fight in the brawl over Dickerson.
I think the Justice Department generally makes two exceptions, and it’s important to bear in mind that we are talking here about Justice Department tradition, not rules or regs — no Justice Department is required to follow tradition. The two are: (a) a statute that unconstitutionally limits the authority of the executive branch (until Obama was elected, I would have said it is inconceivable that DOJ would ever defend the War Powers Resolution if it were ever litigated; now I’m not so sure — if you take seriously some of the things Obama said as a senator, maybe his DOJ would do it); and (b) a statute which the Solicitor General’s office and the Office of Legal Counsel, upon studying all sides of the question, came to view as so clearly unconstitutional that they would decline to defend it, even if there was a plausible argument to be made in its defense.
My problem with Dickerson is that I thought Justice contradicted the spirit of both of these conditions. First, the statute restored executive police power that Miranda had dubiously limited (i.e., before Miranda, a prosecutor was limited only by the Fifth Amendment guarantee against coerced confessions). Second, there was a very strong argument that the statute was constitutional (Congress is primarily in charge of prescribing rules of procedure; Congress is permitted to narrow Supreme Court procedural rulings that are not interpretations of the Constitution; and the Supreme Court had for 30 years said Miranda was not part of the Fifth Amendment but a “prophylaxis” around the Fifth Amendment — and one from which it carved out various exceptions.)
I don’t think a Justice Department would have to defend McCain-Feingold, especially if it were part of an administration that had run on an originalist or strong individual rights platform. (I assume the Bush DOJ defended it because — UGH! — President Bush himself signed it, not because the folks there thought it was a good law.)