What Is Obama Required To Defend?

Tom Garrett worries about the consequences of the Obama decision not to defend Section 3 of DOMA in the courts. Ilya Somin believes the president's "duty to uphold the Constitution supersedes his obligation to enforce federal statutes when the two come into conflict":

The fact that the administration chooses not to defend a federal law doesn’t mean that it won’t have other able defenders. In practice, virtually any significant federal law is likely to be supported by states and/or private parties who have standing to intervene. For example, any of the 45 states that today forbid gay marriage would probably have standing to defend its constitutionality on the grounds that otherwise they might have to extend tax credits and other government benefits to resident couples who have entered into same-sex marriages in other states. If a future Republican administration chooses not to defend the constitutionality of the individual mandate, both state governments who support it and various private parties who benefit from it materially would have standing to intervene. For example, insurance companies support the mandate because it requires people to buy their products and that financial stake in the law is surely sufficient to give them standing.