WASHINGTON — In a significant change of course, President Barack Obama has decided that a federal law against gay marriage is unconstitutional and will no longer defend it in court, the White House announced Wednesday.

Obama’s decision will not have an immediate impact. Attorney General Eric Holder said the president will continue to enforce the Defense of Marriage Act until it’s either clearly struck down by the courts or repealed by Congress, which he’s urged.

That means that an estimated 1,140 laws and policies regarding marriage will remain in place and enforced, and that gay couples who are married in the states where they live will still be denied the federal benefits of marriage, in matters such as Social Security survivor benefits and taxes.

But it signaled a change of track for the administration, and underscored the evolution of the issue over recent years.

The law, passed by a Republican Congress and signed into law by Democratic President Bill Clinton as he sought re-election in 1996, defined marriage as between one man and one woman.

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

“”The Supreme Court has ruled that laws criminalizing homosexual conduct are unconstitutional. Congress has repealed the military’s ‘don’t ask, don’t tell’ policy. Several lower courts have ruled DOMA itself to be unconstitutional.””

The political landscape also has changed. Gay marriage was broadly unpopular when Clinton signed the law, but is much less so now.

The ranks of Americans who think gay marriage should be illegal have dropped from 68 percent in 1996 to 53 percent in 2010, according to a Gallup poll. At the same time, the total of Americans who think it should be legal has risen from 27 percent to 44 percent.

As public opposition has eased, five states have legalized gay marriage since 2004: Massachusetts, Connecticut, Iowa, New Hampshire and Vermont, as well as the District of Columbia. Maryland may soon join them.

Obama’s view also has evolved.

As a candidate in 2008, he endorsed civil unions to guarantee rights for gay couples, but said, “”I believe that marriage is the union between a man and a woman.””

In December, he told The Advocate, a gay and lesbian magazine, that “”like a lot of people, I’m wrestling with this. My attitudes are evolving.””

On Wednesday, White House press secretary Jay Carney said Obama is still debating the question personally. “”He’s grappling with the issue,”” Carney said.

While the law’s supporters criticized Obama’s decision, he didn’t initiate it. Lower court challenges to the law required his response. Indeed, Holder and the Justice Department announced the decision, and the White House spoke only about it when asked, and then only briefly.

Regardless of his personal opinion of marriage and the politics of the issue, Carney said, Obama agrees with Holder that the law’s definition of marriage, excluding gay couples, is unconstitutional and no longer can be defended in court.

Holder said the Justice Department had a long-standing practice of defending the constitutionality of laws as long as “”reasonable arguments”” could be made for them. But he said the department also has refused to defend laws when the president has concluded they’re unconstitutional.

In a letter to House Speaker John Boehner, R-Ohio, Holder said Congress could defend the law itself. But he said the law was intended to discriminate against gays and lesbians.

He said the Congressional Record includes “”numerous expressions reflecting moral disapproval of gays and lesbians and their intimate and family relationships, precisely the kind of stereotype-based thinking and animus”” that the Constitution’s Equal Protection Clause was written to stop.

Holder also said that “”a growing scientific consensus accepts that sexual orientation is a characteristic that is immutable.””

House Judiciary Committee Chairman Lamar Smith, R-Texas, criticized the decision as an abdication of duty.

“”The Justice Department has a responsibility to defend the laws passed by Congress regardless of the personal political views of the president or the attorney general,”” he said.

The law’s supporters weren’t happy, either.

“”DOMA is the law of the land, whether he (Obama) personally likes it or not,”” said Penny Nance, the chief executive of Concerned Women for America, a conservative group. “”For him to circumvent the will of the American people, once again, to impose his latest personal fad into federal law is simply disgraceful.””

But gay rights advocates hailed the decision.

“”This is a monumental decision for the thousands of same-sex couples and their families who want nothing more than the same rights and dignity afforded to other married couples,”” said Joe Solmonese, the president of the Human Rights Campaign, a gay rights group.