By Julia Edwards
WASHINGTON (Reuters) - Eric Holder, the first black U.S. attorney general and one of President Barack Obama's closest allies, will announce on Thursday he is stepping down after a contentious term marked by advances in civil rights and frequent battles with Republicans in Congress.
Holder will remain in office until a successor is nominated and confirmed, the Justice Department said. Obama is scheduled to join him for the announcement at the White House at 4:30 p.m. ET (2030 GMT).
Holder's departure after nearly six years sets up a potentially tense confirmation battle with Republicans in a lame-duck U.S. Senate session scheduled after the Nov. 4 midterm elections, when Republicans hope to capture a Senate majority that would take office in January.
A White House official said Obama has not made a decision on a replacement and will not name a successor on Thursday. Names that have been floated as possible successors include Manhattan U.S. Attorney Preet Bharara, Solicitor General Don Verrilli, former Associate Attorney General Tom Perrelli and Massachusetts Governor Deval Patrick.
Holder was personally close to Obama and unapologetically embodied many of the president's most liberal positions, including support for more gun control, criticism of America's prison system and a desire to try terrorism suspects in civilian instead of military courts.
Despite a drumbeat of Republican criticism since he became chief U.S. prosecutor in 2009, Holder stayed on to be among the longest-serving attorneys general ever and a senior member of Obama's cabinet.
He is one of just three original members of Obama's cabinet who remain, along with Education Secretary Arne Duncan and Agriculture Secretary Tom Vilsack.
Holder made civil rights a cornerstone of his tenure, bringing a series of cases against local police for using excessive force, suing the state of Arizona over a law aimed at Hispanic immigrants and successfully blocking many state voter ID laws before the 2012 election, likening them to Jim Crow-style poll taxes.
He visited Ferguson, Missouri, last month, promising a Justice Department investigation after the shooting death of a black teenager by a white policeman led to violent clashes with police.
While Holder has no immediate plans once he steps down, the Justice Department official said, he has told friends since visiting Ferguson last month that he wants to find a way to help restore trust between law enforcement and minority communities.
Holder built a name more on the people he did not prosecute than on those he did, which is unusual for an attorney general.
The Justice Department did not criminally charge any major Wall Street firm or executive for fraud in connection with the 2007-2009 financial crisis. Holder steered clear of criminal charges against CIA agents involved in waterboarding, an Arizona sheriff investigated for civil rights violations and disgraced cyclist Lance Armstrong, who admitted using performance enhancing drugs.
Among his final projects was a scaling back of the prosecution of nonviolent drug offenders, so that some would no longer face long, mandatory prison sentences.
Holder had previously signalled he planned to step down by the end of the year, and the Justice Department said he finalised his decision at a White House meeting earlier this month.
Holder was a natural choice for attorney general after campaigning for Obama in 2008, when he took on the sensitive job of helping to vet Obama's choices for a vice presidential nominee.
His rounded resume included Ivy League degrees, a job prosecuting corruption and a judgeship in Washington. Holder served in the Justice Department's No. 2 job under Democratic President Bill Clinton.
The Senate confirmed him on a 75-21 vote in 2009, reflecting some Republican uneasiness about Holder's liberal views and about his role in Clinton's pardon of fugitive financier Marc Rich in January 2001.
Holder quickly reassured Republicans when he dropped the Justice Department's corruption case against Ted Stevens, a former Republican senator from Alaska whom lawmakers had sympathy for. Prosecutors in the case improperly withheld evidence from Stevens' defence lawyers.
The honeymoon began to fade during Holder's first month in office in February 2009 when, in a speech on civil rights, he said America was acting as a "nation of cowards." Critics denounced the comment as not sufficiently patriotic.
Later, he infuriated Republicans when he reopened a criminal investigation into the CIA's use of waterboarding and other harsh interrogation methods - the same probe that ended with no prosecutions. Many Republicans thought the CIA's actions were defensible and legal.
Holder's battles with Republicans reached a peak in 2012 when the Republican-led House of Representatives voted largely along party lines to find him in contempt for withholding documents from them.
Obama claimed privilege over the documents about how the Justice Department responded to revelations about a botched anti-gun-trafficking program along the U.S.-Mexico border known as Operation Fast and Furious.
(Additional reporting by Roberta Rampton, Susan Heavey, David Ingram, Aruna Viswanatha)