The Obama administration said Wednesday that it will no longer defend the federal law that bans the recognition of same-sex marriage because it considers the legislation unconstitutional, a sudden and rare reversal.

Gay rights groups hailed the administration's move, saying it will bolster their argument that laws that apply a different standard to people based on sexual orientation are unconstitutional. At least three challenges to the Defense of Marriage Act are working their way through the federal courts.

"When the U.S. government takes this position and then is taking it before the courts of the United States, it seems to me that's a powerful argument to add to the mix," said Gary Buseck, legal director for the Gay and Lesbian Advocates and Defenders.

The move, the latest in a series of political victories for gay rights advocates, was driven by the White House and announced by Attorney General Eric H. Holder Jr. Republicans said the decision was political.

As recently as last month, the Justice Department had vowed to continue going to court to oppose those who were challenging the 1996 Defense of Marriage Act, which defines marriage as between a man and a woman and denies marriage-based federal benefits to same-sex married couples.

But Holder said Wednesday that he and President Obama had determined after an extensive review that the law's key section is unconstitutional. "Given that conclusion, the President has instructed the Department not to defend the statute" in court, Holder said in a statement.

Administration officials said the review was triggered by a court-imposed filing deadline in two new challenges to the law, filed in federal courts in New York and Connecticut.

The change in position - for the time being, at least - could be more symbolic than legal. Although the administration will no longer defend the law, any member of Congress may do so. On Wednesday, opponents of same-sex marriage called on House Speaker John A. Boehner (R-Ohio) to intervene.

Michael Steel, a spokesman for Boehner, declined to say what the speaker will do.

Some opponents of same-sex marriage said the administration's decision could end up helping to preserve the law in court.

"The previous efforts of the Obama administration and DOJ to defend the law were so inadequate as to raise the suspicion that the Justice Department was deliberately throwing the case," said Robert George, a political science professor at Princeton University who opposes same-sex marriage. "Chances are the law will get a robust defense, and I suspect it will withstand constitutional scrutiny."

In the meantime, the law remains in place. Under the measure, same-sex couples who have been legally married can be denied certain federal benefits. For example, gays who work for the federal government cannot extend their health benefits to their spouses.