As explained in this brief (pp. 7-8) filed by former Solicitor General Paul Clement on behalf of the House of Representatives in the Ninth Circuit proceedings in a case challenging the Defense of Marriage Act, it is one thing for the Department of Justice to take the extraordinary step of failing to defend a federal statute that doesn’t intrude on the powers of the executive branch. It is quite another for the Department of Justice to “affirmatively attack” such a statute. According to the brief, that latter course—the course that the Obama administration is pursuing in DOMA litigation—is, “to our knowledge, wholly unprecedented.”
(This SCOTUSblog post by Lyle Denniston, from which I learned of the House’s brief, explains the rather convoluted briefing schedule for this appeal.)
Update (6 p.m.): As a reader reminds me (and as I should have recalled), in Dickerson v. United States the Department of Justice in the Clinton administration argued that 18 U.S.C. 3501 is unconstitutional. The House brief doesn’t address this seeming precedent, so it’s unclear to me whether it would distinguish it in some way. I’ve therefore altered the title of this post by inserting a question mark.