Ted's absolutely right that it's very unusual for an administration to refuse to defend a duly signed federal statute, but it's hardly unprecedented. The closest relatively recent analogue of which I'm aware -- in which the constitutional rule being invoked in refusing to enforce the law is unsettled by the Supreme Court and politically charged -- is the George H.W. Bush administration's decision not to defend federal affirmative-action contracting in Metro Broadcasting. The acting solicitor general for that case was, interestingly, one John Roberts. See this interesting analysis from Marty Lederman.
Traditionally, the Justice Department has declined to defend a law when:
- The law is clearly unconstitutional under intervening Supreme Court decisions (not the case here, at least if the premise is that sexual orientation is entitled to heightened-scrutiny review, which the Court has never determined);
- The law infringes on Presidential powers (not the case here); or
- The President himself has openly stated that he believes the law to be unconstitutional (now the case here -- and likely a key reason why the attorney general invoked the president's position in his letter to the speaker)
Ed Whelan, no friend of the position that DOMA is unconstitutional, observes that "[t]oday's action at least has the modest virtue of bringing [the Obama administration's position] out into the open." Whelan has previously argued, with some force, that "the Obama administration has been sabotaging DOMA litigation from the outset." In general, litigation requires vigorous advocacy, though the aggressive participation of amici in this litigation removes a major obstacle otherwise inherent in an administration decision to give less than a full-throated defense.
The most interesting question to me is that posed by Dale Carpenter: who exactly, apart from the administration, has standing to defend this law? Where standing for an individual plaintiff would be pretty clear for the health-care law or affirmative-action program, it's not self-evident who exactly suffers the requisite harm here, if Congressional leaders aren't allowed to step in.
Also, there's an important point much of the coverage has been missing so far: the administration's position here extends only to Section 3 of the DOMA, which precludes same-sex married couples -- as recognized by their own state -- from receiving federal benefits due to married couples. Holder's letter does not express an opinion on DOMA's application of the Full Faith and Credit Clause to undergird states' decisions not to recognize other states' same-sex marriage licenses.
MORE: Here's an interesting article on the topic from Clinton-era Solicitor General Seth Waxman, related to his decision not to support a Congressional statute intended to overturn Miranda in Dickerson.