President Barack Obama concluded Wednesday that the Defense of Marriage Act is unconstitutional and instructed the Justice Department to stop defending it in court — a major shift in Obama’s stance on an issue that has dogged him throughout his presidency.

The law establishes a federal standard that marriage can only refer to a union between a man and a woman.

“After careful consideration … 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,” Attorney General Eric Holder said in a statement.

“The president has also concluded that [the core provision of] DOMA, as applied to legally married same-sex couples, fails to meet that standard and is therefore unconstitutional. Given that conclusion, the president has instructed the department not to defend the statute in such cases. I fully concur with the president’s determination.”

Obama has come under fire from his left flank over DOMA since he took office: While he has said he opposes the federal ban on gay marriage and that the law should be repealed, he has also maintained that his administration has a legal responsibility to defend it in court. Wednesday’s news means that the Justice Department will no longer defend the law in any current or future cases.

Holder sent a letter to Members of Congress notifying them of Obama’s decision “so Members who wish to defend the statute may pursue that option.”

The attorney general noted that DOMA will still remain in effect unless Congress repeals it or a judge strikes it down, and he said the administration is still obligated to enforce the law. But the difference, he said, is that “this administration will no longer assert its constitutionality in court,” he said.

The law has been challenged in court by gay married couples who charge that they are being unfairly treated because their state-sanctioned marriages are not recognized by the federal government.

White House Press Secretary Jay Carney said Obama’s personal views on gay marriage are “distinct” from his legal opinions.

Obama has said he opposes gay marriage, but in December he told The Advocate, a gay news magazine, that his views on gay marriage are “evolving.”

Obama is still “grappling with the issue” on the personal front, Carney said during a briefing. But his legal views have been consistent because he has always said DOMA is “unnecessary and unfair.”

Carney noted that timing of the president’s decision was tied to a court-imposed deadline in a case that set a precedent for defining DOMA in a way the administration had not done in prior cases.