US President Barack Obama has paid a deeply personal tribute to his Attorney General Eric Holder, as a man with whom he shares bonds of friendship and race said he would resign.
Holder, the first African American to hold the top law enforcement post, is seen as a champion of civil rights by supporters but reviled as an ideologue by Republicans.
He wiped away a tear at a White House ceremony, as Obama hailed his friend as an exemplar of justice.
Holder's departure triggers what could be a divisive Senate confirmation battle for whoever Obama selects as his replacement, particularly as mid-term elections in which Republicans seek to overturn Democratic control of the Senate are just six weeks away.
"This is bittersweet," said Obama, but empathised with the sacrifices made by Holder's family as he served in one of the toughest jobs in government.
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The president said Holder had devoted his life to ensuring that the right to pursue life, liberty and happiness set out in the Declaration of Independence were available to all Americans.
"He believes, as I do, that justice is not just an abstract theory, it's a living and breathing principle," Obama said.
The president hailed Holder's legacy in fighting violent crime and corruption, prosecuting terrorism cases, improving civil rights and safeguarding financial markets.
Holder is one of just three original cabinet members still serving in the job they took when Obama assumed power in 2009, alongside Education Secretary Arne Duncan and Agriculture Secretary Tom Vilsack.
"We have been great colleagues, but the bonds between us are much deeper than that," Holder told Obama.
"In good times and bad, in things personal and in things professional, you have been there for me."
Only three other US attorneys general have served longer than Holder, whose tenure was notable for significant inroads made in the civil rights arena, including gains in rights and benefits for same-sex couples and reductions in prison sentences for certain drug offences.
