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.

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.