WASHINGTON – President Obama ordered his administration on Wednesday to stop defending the constitutionality of a federal law that bans recognition of gay marriage, a policy reversal that could have major implications for the rights and benefits of gay couples and reignite an emotional debate for the 2012 presidential campaign.

Obama still is “grappling” with his personal views on whether gays should be allowed to marry but has long opposed the federal law as unnecessary and unfair, said spokesman Jay Carney.

First word of the change came not from the White House, but from the Justice Department. Attorney General Eric Holder announced that Obama had concluded the 15-year-old Defense of Marriage Act, or DOMA, was legally indefensible.

The decision was immediately welcomed by gay-rights organizations and vilified by those on the other side. Some Democrats in Congress praised the decision, while it drew criticism from some Republicans and the office of their leader, House Speaker John Boehner, all surely a preview of coming political debate over the latest development in the long-running national conversation about gay rights.

The outcome of that debate could have enormous impact because federal laws and regulations confer more than 1,000 rights or benefits on those who are married, most involving taxpayer money — Social Security survivor benefits, family and medical leave, equal compensation as federal employees, and immigration rights.

“Much of the legal landscape has changed in the 15 years since Congress passed DOMA,” Holder said in a statement explaining the decision.

And the social landscape has changed as well.

Since the law was passed in 1996, five states and the District of Columbia have approved gay marriage, and others allow civil unions. An Associated Press-National Constitution Center Poll conducted last August found 52 percent of Americans think the federal government should give legal recognition to marriages between couples of the same sex.

Thirty states have constitutional amendments banning gay marriage. Same-sex marriage is legal in Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and Washington, D.C.

The White House framed Obama’s decision as one brought on by a legal deadline in one of several federal court cases challenging the constitutionality of the law that defines marriage as only between a man and a woman.

But Sen. Jim DeMint, R-S.C., speculated that Obama’s decision was motivated more by political considerations. “It’s only in the run-up to re-election that he’s suddenly changed his mind,” DeMint said.

Obama’s reversal on this law had long been sought by gays, who overwhelmingly voted for his election in 2008.

The Justice Department had defended the act in court until now. But Holder said Obama concluded the law fails to meet a rigorous standard under which courts view with suspicion any laws targeting minority groups, such as gays, who have suffered a history of discrimination — a stricter standard of scrutiny than the department has applied in the past.

Looking back to Congress’ debate on the legislation, Holder said it was clear that there were “numerous expressions reflecting moral disapproval of gays and lesbians and their intimate and family relationships — precisely the kind of stereotype-based thinking and animus the (Constitution’s) Equal Protection Clause is designed to guard against.”

Gay-rights activists noted that the president’s move came just two months after Congress, urged on by the administration, voted to repeal the “don’t ask, don’t tell” policy that prevented gays from serving openly in the military.

“This major turn should be a final nail in the coffin for the different treatment of gay and non-gay people by the federal government,” said law professor Suzanne Goldberg, director of Columbia University’s Center for Gender and Sexuality Law.

Rea Carey, executive director of the National Gay and Lesbian Task Force, called the change “a tremendous step toward recognizing our common humanity and ending an egregious injustice against thousands of loving, committed couples who simply want the protections, rights and responsibilities afforded other married couples.”

On the other side of the debate, reaction was vehement.

“On the one hand, this is a truly shocking extra-constitutional power grab in declaring gay people are a protected class,” said Maggie Gallagher of the conservative National Organization for Marriage. “The good news is this now clears the way for the House to intervene and to get lawyers in the courtroom who actually want to defend the law, and not please their powerful political special interests.”

Boehner’s spokesman, Michael Steel, issued a statement faulting Obama for stirring up the issue “while Americans want Washington to focus on creating jobs and cutting spending.”

For now, the law remains on the books while challenges work through the courts.

Among those affected by DOMA were a lesbian couple from New York City, Edith Windsor and Thea Spyer. After four decades together they married in Canada in 2007, and that marriage was recognized in New York.

However, it was not recognized by the federal government. One result, according to a lawsuit filed by the American Civil Liberties Union, was a $350,000 federal tax on Spyer’s estate when she died in 2009 that Windsor would not have had to pay if she were in a heterosexual marriage.

Windsor said she was elated by the Justice Department announcement.

“My only regret is that my beloved late spouse … isn’t here today to share in this historic moment,” she said. “But in my heart, I feel that she knows.”