Washington (CNN) -- President Barack Obama has ordered the Justice Department to stop defending the constitutionality of the Defense of Marriage Act, which defines marriage for federal purposes as only between a man and woman, according to a statement Wednesday from Attorney General Eric Holder.
"The president has concluded that given a number of factors, including a documented history of discrimination, classifications based on sexual orientation should be subject to a more heightened standard of scrutiny," Holder said.
The key provision in the law "fails to meet that standard and is therefore unconstitutional."
"Given that conclusion, the president has instructed the (Justice Department) not to defend the statute" in two pending cases in New York, Holder said. "I fully concur with the president's determination."
Obama has previously expressed his personal opposition to the Defense of Marriage Act but had never stated an opinion relating to its constitutionality.
The administration had a March 11 deadline to respond to two lawsuits against the measure in New York. The 2nd U.S. Circuit Court of Appeals -- which includes New York -- is the only federal circuit to have never decided the basic legal question of whether a law discriminates against gay men and lesbians.
Republicans immediately ripped the White House's decision, calling it a distraction at a time when they said the focus needs to be on the economy.
"While Americans want Washington to focus on creating jobs and cutting spending,the president will have to explain why he thinks now is the appropriate time to stir up a controversial issue that sharply divides the nation," said Michael Steel, spokesman for House Speaker John Boehner, R-Ohio.
Representative Lamar Smith, R-Texas, chairman of the House Judiciary Committee, called the move a "transparent attempt to shirk the (Justice) Department's duty to defend the laws passed by Congress."
"This is the real politicization of the Justice Department -- when the personal views of the president override the government's duty to defend the law of the land," Smith said.
White House press secretary Jay Carney said the administration had to make a decision before the court-imposed deadline. He stressed, however, that the law will continue to be enforced.
House Minority Leader Nancy Pelosi, D-California, praised the administration's decision, calling it a key step forward in the push for equal rights for gays and lesbians.
It "is a victory for civil rights, fairness and equality," Pelosi said. "I commend (Obama) for taking this bold step forward to ensure the federal government is no longer in the business of defending an indefensible statute."
Joe Solmonese, president of the progressive Human Rights Campaign, also praised the administration's course of action, calling it "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."
"We applaud (Obama) for fulfilling his oath to defend critical constitutional principles," Solmonese said.
The Defense of Marriage Act was passed in 1996 by the GOP-controlled Congress and signed into law by President Bill Clinton. It bars federal recognition of same-sex marriages and says states cannot be forced to recognize such marriages from other states.
In July, a federal judge in Massachusetts became the first to rule the law unconstitutional. U.S. District Judge Joseph Tauro said that "irrational prejudice plainly never constitutes a legitimate government interest."
The administration's decision comes on the heels of other major developments in the struggle over gay and lesbian rights. In December, Obama signed legislation that will repeal the controversial "don't ask, don't tell" policy and bring an end to the ban on openly gay men and women serving in the armed forces.
Courts in California are considering a legal challenge to Proposition 8, an initiative narrowly approved by that state's voters in 2008. It defines marriage as a union between a man and a woman.
Same-sex marriage is legal in five states -- Massachusetts, Connecticut, Vermont, Iowa and New Hampshire -- and in the District of Columbia. Civil unions are permitted in New Jersey.
CNN's Alan Silverleib, Tom Cohen and Bill Mears contributed to this report