Congress could, but it's not their responsibility. The executive branch is required to defend the laws that Congress passes.
Nope. It is required to enforce
them, not to defend
them, and indeed it continues to enforce them. There is no constitutional responsibility incumbent upon the executive branch to defend laws in court. That responsibility is one which is typically delegated to it by the legislative branch, but the prerogative to do so remains with the legislative branch.
Further, the executive branch of the government has informed the judicial branch of the government that it supports removal of the law. This helps in one particular, tricky respect: judicial deference. Normally, the judicial branch tends to be somewhat reluctant to strike down laws and actions of the other branches without really compelling reasons. However, when at least one of the other branches agrees
that it needs to happen, it becomes much more likely that the judicial branch will rule the law unconstitutional. This has been the case in civil-rights issues in the past.
There's one other thing to keep in mind: should Congress agree to defend the law (as it seems likely that it will), then it will face not only the burden of showing why DOMA is constitutional (the burden the administration already faced and obviously did not believe was the case), but of showing why the administration is wrong in its appraisal of the Constitution. They will also face an uphill battle getting the courts to review the issue in light of the rational basis test.
Finally, should Congress bring suit, then it will be forced to defend itself using the legislative record, which is full of bitter and bare-faced anti-gay sentiment. Historically, the presence of this sentiment has tended to turn the opinions of judges and legislators alike towards the pro-gay side. They'll be facing an uphill battle in more ways than one, in short. And that's not even getting into the political implications of what the Democrats will most certainly frame as abandoning the nothing-but-jobs rhetoric in favor of pandering to the hard right!
edited 1st Mar '11 7:43:34 PM by Chalkos