WASHINGTON — Attorney General Eric H. Holder Jr. is resigning, the Justice Department said Thursday, but plans to remain in office until a successor is confirmed.

Mr. Holder, the 82nd attorney general and the first African-American to serve in that position, had previously said he planned to leave office by the end of this year.

Particularly in President Obama’s second term, Mr. Holder has been the most prominent liberal voice of the administration, leading its push for same-sex marriage and voting rights.

After the recent shooting of an unarmed black teenager by a white police officer, Mr. Holder volunteered to go to Ferguson, Mo., as the administration’s emissary.

Nobody else in the administration, Obama included, has done nearly as much to protect and support the rights of African Americans.

A senior White House official said the president was “a long way” from announcing Mr. Holder’s replacement. Frequently mentioned candidates for the job include: Kathryn Ruemmler, the former White House counsel; Gov. Deval Patrick of Massachusetts; Solicitor General Donald B. Verrilli Jr.; former Gov. Jennifer M. Granholm of Michigan; Senator Sheldon Whitehouse, Democrat of Rhode Island; and Loretta E. Lynch, the United States attorney in Brooklyn.

The Justice Department said Mr. Holder finalized his plans to leave in an hourlong conversation with Mr. Obama at the White House over Labor Day weekend. A formal announcement was scheduled for Thursday afternoon at the White House.

Mr. Holder is a former federal prosecutor and Superior Court judge in Washington and served as the deputy attorney general under Attorney General Janet Reno.

A child of the civil rights era, Mr. Holder was shaped by images of violence in Selma, Ala. He joined sit-ins at Columbia University, where protesters renamed an office after Malcolm X. He has frequently spoken in personal terms about civil rights, including about his experiences as a prosecutor in Washington and as a college student, when he was stopped by the police.

As attorney general, he has pushed to change what he sees as fundamental inequities in the criminal justice system. He told prosecutors not to seek long sentences for low-level crime, and he pushed to eliminate those sentences for nonviolent drug crimes. He has joined with liberal Democrats and libertarian-minded Republicans to advocate for the most sweeping liberalization of sentencing laws since the beginning of the War on Drugs.

Early this week, Mr. Holder announced that the federal prison population declined this year for the first time since 1980. He projected the decline would continue for the next two years.

But his record on civil liberties has earned him fewer accolades. He authorized subpoenas directed at journalists and approved the Central Intelligence Agency’s killing of Anwar al-Awlaki, an American citizen working with the Al Qaeda branch in Yemen. Mr. Holder signed off on the National Security Agency’s authority to sweep up the phone records of millions of Americans not charged with any crime.

Mr. Holder was among Mr. Obama’s first nominees to the cabinet. The president-elect announced his selection on the same day he announced his choice of Hillary Rodham Clinton to be secretary of state.

The new attorney general quickly became an outspoken voice on issues that remained sensitive for a first-term president, especially race. While Mr. Obama only rarely confronted issues of race and discrimination, Mr. Holder emerged as a forceful advocate in those areas.

Mr. Holder was also a lightning rod, attracting partisan venom in Congress that might otherwise have been aimed more squarely at the president. He was attacked by critics in the wake of a botched gun program along the border with Mexico. Republican lawmakers in the House held Mr. Holder in contempt of Congress.

With Mr. Holder’s imminent departure, Mr. Obama must now find a successor who can follow through on Mr. Holder’s initiatives for the remainder of the president’s second term.

Mr. Holder’s successor will play a central role shaping the legal policies behind the war Mr. Obama is waging in Iraq and Syria. The new attorney general will also have a big hand in helping enact new executive orders that Mr. Obama has announced, including a change in immigration policy that is expected after the midterm elections.

Finding someone who can win confirmation in the Senate may be a serious challenge for the president, especially if Republicans gain control of the chamber in November. Republicans are likely to try and hold up any nominee they perceive as too liberal, even if Democrats barely maintain their hold on the Senate.

Inside the Justice Department, many career prosecutors see Mr. Holder’s push to prosecute terrorism suspects in criminal courts as his most enduring legacy. While the military tribunals at Guantánamo Bay have languished, the Justice Department has won several high-profile convictions and lengthy sentences in terrorism cases.

Through it all, Mr. Holder built one of the closest personal relationships with Mr. Obama of any members of the president’s cabinet. Mr. Holder and his wife regularly spent their summer vacations on Martha’s Vineyard at the same time Mr. Obama did, with the two couples sharing dinner and spending time together on the island.