In September, the Justice Department sued North Carolina, challenging voting restrictions that Holder said disproportionately affected "people who are young, people of color, people who are poor." The lawsuit followed a 2013 U.S. Supreme Court ruling that invalidated part of the 1965 Voting Rights Act that opened the polls to millions of black Americans. The department had previously sued Texas on similar grounds.

Guantanamo, Terrorism

The steps on social policy come as Holder begins his sixth year as the country's top lawyer amid criticism from Republican lawmakers, who have blocked several of his previous policies. Holder's plan to try accused 9/11 plotter Khalid Sheikh Mohammed in a civilian court was rejected by Congress, as was the administration's plan to close the detention camp in Guantanamo Bay, Cuba.

Holder was also the first sitting cabinet member to be held in contempt by lawmakers when House Republicans accused him of refusing to turn over information on his department's botched Fast and Furious operation, a probe that allowed illegal gun purchases in the U.S. in an effort to link the weapons to Mexican gangs. He angered Democrats and Republicans alike over his department's secret subpoena of Associated Press phone records in an investigation of leaks.

Exit Talks

In his remarks on Satursday, Holder, the first black U.S. attorney general, linked the fight over gay marriage to the struggle of civil rights advocates in the 1950s and 1960s.

"As all-important as the fight against racial discrimination was then, and remains today, know this: My commitment to confronting discrimination based on sexual orientation or gender identity runs just as deep," he said.

The policy change drew criticism from the National Organization for Marriage, a group seeking to block legalization of same-sex unions.

"It's more utter lawlessness from the Obama administration," said Brian Brown, the group's president.

The new policy follows a U.S. Supreme Court decision last June that struck down the federal Defense of Marriage Act's definition of marriage as a heterosexual union, a provision that had denied legally married gay couples federal benefits. On the same day, the court let stand a ruling that California's ban on same-sex marriage was unconstitutional.

17 States

Gay marriage is legal in 17 U.S. states and the District of Columbia.

The Justice Department will also provide death and educational benefits, through the Public Safety Officers' Benefits Program, to same-sex spouses of law-enforcement officers killed in the line of duty.

Holder's move comes as polls have shown a growing public acceptance in America of gay marriage. In a Gallup Poll released last July, 52 percent of Americans backed laws permitting gay marriage, up from 27 percent in 1996.

Obama, after publicly saying he didn't favor gay marriage, reversed himself in a televised interview in 2012, saying he thought "same-sex couples should be able to get married."